Rails HEAD Gemfile: use master autoprefixer

Author: glebm

test/gemfiles/rails_head.gemfile

@@ -9,7 +9,7 @@ gem 'sprockets', github: 'rails/sprockets'
 gem 'sprockets-rails', github: 'rails/sprockets-rails'
 gem 'sass-rails', github: 'rails/sass-rails', branch: 'master'
 
-gem 'autoprefixer-rails', github: 'glebm/autoprefixer-rails', branch: 'rails5-compat'
+gem 'autoprefixer-rails', github: 'ai/autoprefixer-rails'
 
 source 'https://rails-assets.org' do
   gem 'rails-assets-tether', '>= 1.1.0'