Update rubocop, add appraisal gem

Appraisals

@@ -0,0 +1,14 @@
+appraise 'rails-51' do
+  gem 'actionpack', '~> 5.1'
+  gem 'railties', '~> 5.1'
+end
+
+appraise 'rails-50' do
+  gem 'actionpack', '~> 5.0'
+  gem 'railties', '~> 5.0'
+end
+
+appraise 'rails-42' do
+  gem 'actionpack', '~> 4.2'
+  gem 'railties', '~> 4.2'
+end