Make simplecov a source development dependency only (#1418)

Remove the need for downstream packaging to either require or patch
out simplecov from existing files by making it an optional development
dependency.

Gemfile

@@ -37,11 +37,9 @@ group :development do
     gem 'rexml'
   end
 
-  if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.3.0')
-    gem 'mime-types', '< 3.4.0'
-  end
-
   gem 'pry'
+  gem 'simplecov'
+  gem 'simplecov-lcov'
 end
 
 group :plugins do