Check if it is working outside of group :assets in production.

Gemfile

@@ -12,7 +12,6 @@ gem 'json'
 # Gems used only for assets and not required
 # in production environments by default.
 group :assets do
-  gem 'autoprefixer-rails'
   gem 'sass-rails' #, github: 'rails/sass-rails'
   gem 'coffee-rails'
   gem 'coffee-script-source'
@@ -24,6 +23,8 @@ group :assets do
   gem 'eco'
 end
 
+gem 'autoprefixer-rails'
+
 gem 'omniauth'
 gem 'omniauth-twitter'
 gem 'omniauth-facebook'