Remove coffee-script gem

Gemfile

@@ -14,7 +14,6 @@ gem 'yajl-ruby', require: false
 group :app do
   gem 'browser'
   gem 'chunky_png'
-  gem 'coffee-script'
   gem 'erubi'
   gem 'image_optim_pack', platforms: :ruby
   gem 'image_optim'

Gemfile.lock

@@ -14,10 +14,6 @@ GEM
     byebug (11.1.3)
     chunky_png (1.4.0)
     coderay (1.1.3)
-    coffee-script (2.4.1)
-      coffee-script-source
-      execjs
-    coffee-script-source (1.12.2)
     concurrent-ruby (1.1.10)
     daemons (1.4.1)
     erubi (1.12.0)
@@ -146,7 +142,6 @@ DEPENDENCIES
   better_errors
   browser
   chunky_png
-  coffee-script
   erubi
   html-pipeline
   image_optim