Update testing matrix

Appraisals

@@ -27,13 +27,3 @@ appraise 'rails-51' do
   gem 'actionpack', '~> 5.1.0'
   gem 'railties', '~> 5.1.0'
 end
-
-appraise 'rails-50' do
-  gem 'actionpack', '~> 5.0.0'
-  gem 'railties', '~> 5.0.0'
-end
-
-appraise 'rails-42' do
-  gem 'actionpack', '~> 4.2.0'
-  gem 'railties', '~> 4.2.0'
-end