Add rails 6.1 support

gemfiles/rails_61.gemfile

@@ -2,10 +2,8 @@
 
 source "https://rubygems.org"
 
-git "https://github.com/rails/rails.git" do
-  gem "actionpack", "~> 6.1.0.alpha"
-  gem "railties", "~> 6.1.0.alpha"
-end
+gem "actionpack", "~> 6.1.0"
+gem "railties", "~> 6.1.0"
 
 group :development do
   gem "appraisal", "~> 2.2"