Update rubocop, add appraisal gem

README.md

@@ -507,7 +507,7 @@ Yes, it's threadsafe too.
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies.
-Then, run `rake spec` to run the tests.
+Then, run `appraisal rake spec` to run the tests.
 You can also run `bin/console` for an interactive prompt that will allow you to experiment.
 
 To install this gem onto your local machine, run `bundle exec rake install`.
@@ -515,16 +515,6 @@ To release a new version, update the version number in `version.rb`,
 and then run `bundle exec rake release`, which will create a git tag for the version,
 push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org).
 
-### Different Rails versions
-
-To setup development for specific major Rails version use:
-
-```
-RAILS=5 bundle install
-# or
-RAILS=5 bundle update
-```
-
 ## Contributing
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/telegram-bot-rb/telegram-bot.