Relax Rails version dependency

We don't need anymore to lock this extension to Rails 5.2.1
since solidus core is now compatible with it.

ref solidusio/solidus#2992

Gemfile

@@ -5,7 +5,6 @@ gem "solidus", github: "solidusio/solidus", branch: branch
 
 if branch == 'master' || branch >= "v2.0"
   gem "rails-controller-testing", group: :test
-  gem "rails", "<= 5.2.1"
 else
   gem "rails_test_params_backport", group: :test
   gem "rails", "~> 4.2.7"