Set TargetRubyVersion in .rubocop.yml
Set TargetRubyVersion in .rubocop.yml

This will fix the following Rubocop offense:

```
statesman.gemspec:20:32: C: Gemspec/RequiredRubyVersion: required_ruby_version (2.2, declared in statesman.gemspec) and TargetRubyVersion (2.1, declared in .rubocop.yml) should be equal.
  spec.required_ruby_version = '>= 2.2'
                               ^^^^^^^^
```

timothyp/statesman,timothyp/statesman,gocardless/statesman,gocardless/statesman