Support for :credentials on Rails v5.2.x. (#60)

gemfiles/rails_51.gemfile

@@ -14,7 +14,7 @@ group :development do
   gem "rspec", "~> 3.5.0"
   gem "rspec-its", "~> 1.1.0"
   gem "rspec-rails", "~> 3.5.0"
-  gem "rubocop", "~> 0.51.0"
+  gem "rubocop", "~> 0.52.1"
   gem "coveralls", "~> 0.8.2", require: false
 end