diff --git a/Gemfile b/Gemfile
index 62104de3a23..7d39c4aff04 100644
--- a/Gemfile
+++ b/Gemfile
@@ -14,23 +14,14 @@ if rails_master?
   gem 'rails', git: 'https://github.com/rails/rails.git'
   gem 'seed-fu', git: 'https://github.com/SamSaffron/seed-fu.git', branch: 'discourse'
 else
-  # Rails 5 is going to ship with Action Cable, we have no use for it as
-  # we already ship MessageBus, AC introduces dependencies on Event Machine,
-  # Celluloid and Faye Web Sockets.
-  #
-  # Note this means upgrading Rails is more annoying, to do so, comment out the
-  # explicit dependencies, and add gem 'rails', bundle update rails and then
-  # comment back the explicit dependencies. Leaving this in a comment till we
-  # upgrade to Rails 5
-  #
-  # gem 'activesupport'
-  # gem 'actionpack'
-  # gem 'activerecord'
-  # gem 'actionmailer'
-  # gem 'activejob'
-  # gem 'railties'
-  # gem 'sprockets-rails'
-  gem 'rails', '~> 4.2'
+  gem 'actionmailer', '~> 5.1'
+  gem 'actionpack', '~> 5.1'
+  gem 'actionview', '~> 5.1'
+  gem 'activemodel', '~> 5.1'
+  gem 'activerecord', '~> 5.1'
+  gem 'activesupport', '~> 5.1'
+  gem 'railties', '~> 5.1'
+  gem 'sprockets-rails'
   gem 'seed-fu', '~> 2.3.5'
 end
 
@@ -55,7 +46,7 @@ gem 'barber'
 
 gem 'message_bus'
 
-gem 'rails_multisite'
+gem 'rails_multisite', '~> 1.1.0.rc4'
 
 gem 'fast_xs'
 
@@ -120,8 +111,6 @@ group :test do
   gem 'webmock', require: false
   gem 'fakeweb', '~> 1.3.0', require: false
   gem 'minitest', require: false
-  # TODO: Remove once we upgrade to Rails 5.
-  gem 'test_after_commit'
 end
 
 group :test, :development do
diff --git a/Gemfile.lock b/Gemfile.lock
index 0dfb26dc5b1..c717e440f2e 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,41 +1,40 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    actionmailer (4.2.9)
-      actionpack (= 4.2.9)
-      actionview (= 4.2.9)
-      activejob (= 4.2.9)
+    actionmailer (5.1.3)
+      actionpack (= 5.1.3)
+      actionview (= 5.1.3)
+      activejob (= 5.1.3)
       mail (~> 2.5, >= 2.5.4)
-      rails-dom-testing (~> 1.0, >= 1.0.5)
-    actionpack (4.2.9)
-      actionview (= 4.2.9)
-      activesupport (= 4.2.9)
-      rack (~> 1.6)
-      rack-test (~> 0.6.2)
-      rails-dom-testing (~> 1.0, >= 1.0.5)
+      rails-dom-testing (~> 2.0)
+    actionpack (5.1.3)
+      actionview (= 5.1.3)
+      activesupport (= 5.1.3)
+      rack (~> 2.0)
+      rack-test (~> 0.6.3)
+      rails-dom-testing (~> 2.0)
       rails-html-sanitizer (~> 1.0, >= 1.0.2)
-    actionview (4.2.9)
-      activesupport (= 4.2.9)
+    actionview (5.1.3)
+      activesupport (= 5.1.3)
       builder (~> 3.1)
-      erubis (~> 2.7.0)
-      rails-dom-testing (~> 1.0, >= 1.0.5)
+      erubi (~> 1.4)
+      rails-dom-testing (~> 2.0)
       rails-html-sanitizer (~> 1.0, >= 1.0.3)
     active_model_serializers (0.8.3)
       activemodel (>= 3.0)
-    activejob (4.2.9)
-      activesupport (= 4.2.9)
-      globalid (>= 0.3.0)
-    activemodel (4.2.9)
-      activesupport (= 4.2.9)
-      builder (~> 3.1)
-    activerecord (4.2.9)
-      activemodel (= 4.2.9)
-      activesupport (= 4.2.9)
-      arel (~> 6.0)
-    activesupport (4.2.9)
+    activejob (5.1.3)
+      activesupport (= 5.1.3)
+      globalid (>= 0.3.6)
+    activemodel (5.1.3)
+      activesupport (= 5.1.3)
+    activerecord (5.1.3)
+      activemodel (= 5.1.3)
+      activesupport (= 5.1.3)
+      arel (~> 8.0)
+    activesupport (5.1.3)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
       i18n (~> 0.7)
       minitest (~> 5.1)
-      thread_safe (~> 0.3, >= 0.3.4)
       tzinfo (~> 1.1)
     addressable (2.5.1)
       public_suffix (~> 2.0, >= 2.0.2)
@@ -43,7 +42,7 @@ GEM
       activerecord (>= 3.2, < 6.0)
       rake (>= 10.4, < 13.0)
     ansi (1.5.0)
-    arel (6.0.4)
+    arel (8.0.0)
     ast (2.3.0)
     aws-sdk (2.5.3)
       aws-sdk-resources (= 2.5.3)
@@ -100,6 +99,7 @@ GEM
       jquery-rails (>= 1.0.17)
       railties (>= 3.1)
     ember-source (2.13.3)
+    erubi (1.6.1)
     erubis (2.7.0)
     excon (0.56.0)
     execjs (2.7.0)
@@ -242,40 +242,28 @@ GEM
     public_suffix (2.0.5)
     puma (3.9.1)
     r2 (0.2.6)
-    rack (1.6.8)
+    rack (2.0.3)
     rack-mini-profiler (0.10.5)
       rack (>= 1.2.0)
     rack-openid (1.3.1)
       rack (>= 1.1.0)
       ruby-openid (>= 2.1.8)
-    rack-protection (1.5.3)
+    rack-protection (2.0.0)
       rack
     rack-test (0.6.3)
       rack (>= 1.0)
-    rails (4.2.9)
-      actionmailer (= 4.2.9)
-      actionpack (= 4.2.9)
-      actionview (= 4.2.9)
-      activejob (= 4.2.9)
-      activemodel (= 4.2.9)
-      activerecord (= 4.2.9)
-      activesupport (= 4.2.9)
-      bundler (>= 1.3.0, < 2.0)
-      railties (= 4.2.9)
-      sprockets-rails
-    rails-deprecated_sanitizer (1.0.3)
-      activesupport (>= 4.2.0.alpha)
-    rails-dom-testing (1.0.8)
-      activesupport (>= 4.2.0.beta, < 5.0)
-      nokogiri (~> 1.6)
-      rails-deprecated_sanitizer (>= 1.0.1)
+    rails-dom-testing (2.0.3)
+      activesupport (>= 4.2.0)
+      nokogiri (>= 1.6)
     rails-html-sanitizer (1.0.3)
       loofah (~> 2.0)
-    rails_multisite (1.0.6)
-      rails (> 4.2, < 5)
-    railties (4.2.9)
-      actionpack (= 4.2.9)
-      activesupport (= 4.2.9)
+    rails_multisite (1.1.0.rc4)
+      activerecord (> 4.2, < 6)
+      railties (> 4.2, < 6)
+    railties (5.1.3)
+      actionpack (= 5.1.3)
+      activesupport (= 5.1.3)
+      method_source
       rake (>= 0.8.7)
       thor (>= 0.18.1, < 2.0)
     rainbow (2.2.2)
@@ -356,7 +344,7 @@ GEM
     shoulda-context (1.2.2)
     shoulda-matchers (2.8.0)
       activesupport (>= 3.0.0)
-    sidekiq (5.0.3)
+    sidekiq (5.0.4)
       concurrent-ruby (~> 1.0)
       connection_pool (~> 2.2, >= 2.2.0)
       rack-protection (>= 1.5.0)
@@ -371,8 +359,6 @@ GEM
       activesupport (>= 4.0)
       sprockets (>= 3.0.0)
     stackprof (0.2.10)
-    test_after_commit (1.1.0)
-      activerecord (>= 3.2)
     thor (0.19.4)
     thread_safe (0.3.6)
     tilt (2.0.7)
@@ -398,7 +384,13 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  actionmailer (~> 5.1)
+  actionpack (~> 5.1)
+  actionview (~> 5.1)
   active_model_serializers (~> 0.8.3)
+  activemodel (~> 5.1)
+  activerecord (~> 5.1)
+  activesupport (~> 5.1)
   annotate
   aws-sdk
   barber
@@ -464,8 +456,8 @@ DEPENDENCIES
   r2 (~> 0.2.5)
   rack-mini-profiler
   rack-protection
-  rails (~> 4.2)
-  rails_multisite
+  rails_multisite (~> 1.1.0.rc4)
+  railties (~> 5.1)
   rake
   rb-fsevent
   rb-inotify (~> 0.9)
@@ -486,8 +478,8 @@ DEPENDENCIES
   shoulda
   sidekiq
   simple-rss
+  sprockets-rails
   stackprof
-  test_after_commit
   thor
   tilt
   uglifier