Add rubocop to our build. (#5004)

db/migrate/20150713203955_enlarge_users_email_field.rb

@@ -1,8 +1,8 @@
 class EnlargeUsersEmailField < ActiveRecord::Migration
   def up
-    change_column :users, :email, :string, :limit => 513
+    change_column :users, :email, :string, limit: 513
   end
   def down
-    change_column :users, :email, :string, :limit => 128
+    change_column :users, :email, :string, limit: 128
   end
 end