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

Appraisals

@@ -1,26 +1,31 @@
-appraise 'rails-60' do
+appraise 'rails-61' do
   git 'https://github.com/rails/rails.git' do
-    gem 'actionpack', '~> 6.0.0.alpha'
-    gem 'railties', '~> 6.0.0.alpha'
+    gem 'actionpack', '~> 6.1.0.alpha'
+    gem 'railties', '~> 6.1.0.alpha'
   end
 end
 
+appraise 'rails-60' do
+  gem 'actionpack', '~> 6.0.0'
+  gem 'railties', '~> 6.0.0'
+end
+
 appraise 'rails-52' do
-  gem 'actionpack', '~> 5.2.0.beta2'
-  gem 'railties', '~> 5.2.0.beta2'
+  gem 'actionpack', '~> 5.2.0'
+  gem 'railties', '~> 5.2.0'
 end
 
 appraise 'rails-51' do
-  gem 'actionpack', '~> 5.1'
-  gem 'railties', '~> 5.1'
+  gem 'actionpack', '~> 5.1.0'
+  gem 'railties', '~> 5.1.0'
 end
 
 appraise 'rails-50' do
-  gem 'actionpack', '~> 5.0'
-  gem 'railties', '~> 5.0'
+  gem 'actionpack', '~> 5.0.0'
+  gem 'railties', '~> 5.0.0'
 end
 
 appraise 'rails-42' do
-  gem 'actionpack', '~> 4.2'
-  gem 'railties', '~> 4.2'
+  gem 'actionpack', '~> 4.2.0'
+  gem 'railties', '~> 4.2.0'
 end