require asset gems in development too

Gemfile

@@ -31,7 +31,7 @@ gem 'createsend', '~> 4.0'
 gem 'acme-client'
 gem 'highline', :require => true
 
-group :assets do
+group :development, :assets do
   gem 'sass-rails', '~> 5.0'
   gem 'uglifier', '>= 1.3.0'
   gem 'coffee-rails', '~> 4.2'