Use Rails 6-1-stable branch

The fix has been merged, but not released yet

Gemfile

@@ -5,7 +5,7 @@ gemspec
 
 rails_version = ENV.fetch("RAILS_VERSION", 6.1).to_f
 if rails_version.to_s.match?(/6.1/)
-  gem "rails", git: "https://github.com/fatkodima/rails", branch: "fix-polymorphic-index-compat-6-1"
+  gem "rails", git: "https://github.com/rails/rails", branch: "6-1-stable"
 else
   gem "rails", "~> #{rails_version}.0"
 end