Require tests against rails 6.0, run tests against rails 6.1.alpha

Rails 6.0 requires ruby >= 2.5, so ruby 2.4 is not tested by default
and replaced with 2.6

gemfiles/rails_52.gemfile

@@ -2,8 +2,8 @@
 
 source "https://rubygems.org"
 
-gem "actionpack", "~> 5.2.0.beta2"
-gem "railties", "~> 5.2.0.beta2"
+gem "actionpack", "~> 5.2.0"
+gem "railties", "~> 5.2.0"
 
 group :development do
   gem "appraisal", "~> 2.2"