Update rubocop, add appraisal gem

Gemfile

@@ -2,24 +2,11 @@ source 'https://rubygems.org'
 gemspec
 
 group :development do
-  case ENV['RAILS']
-  when '5_1'
-    gem 'railties', '5.1.0'
-    gem 'actionpack', '5.1.0'
-  when '5'
-    gem 'railties', '5.0.2'
-    gem 'actionpack', '5.0.2'
-  when '4'
-    gem 'railties', '~> 4.2'
-    gem 'actionpack', '~> 4.2'
-  else
-    gem 'railties'
-    gem 'actionpack'
-  end
+  gem 'appraisal', '~> 2.2'
 
-  gem 'sdoc', '~> 0.4.1'
   gem 'pry', '~> 0.10.1'
   gem 'pry-byebug', '~> 3.2.0'
+  gem 'sdoc', '~> 0.4.1'
 
   gem 'telegram-bot-types', '~> 0.3.0'
 
@@ -27,7 +14,7 @@ group :development do
   gem 'rspec-its', '~> 1.1.0'
   gem 'rspec-rails', '~> 3.5.0'
 
-  gem 'rubocop', '~> 0.37.0'
+  gem 'rubocop', '~> 0.51.0'
 
   gem 'coveralls', '~> 0.8.2', require: false
 end