Replace therubyracer with mini_racer

The therubyracer gem is no longer maintained and the suggestion is to
just use mini_racer instead.

See rubyjs/therubyracer#462

Gemfile

@@ -21,7 +21,7 @@ gem 'rails', rails
 ###############################################
 # The following gems are also needed to test within the Rails env
 gem 'jquery-rails'
-gem 'therubyracer'
+gem 'mini_racer'
 gem 'turbolinks'
 ###############################################