Use rubocop latest, drop support for Ruby 2.1, 2.2

[olleolleolle]

This PR updates the Gemfile use any (aka latest release of) RuboCop.

• Gemspec: Update the lowest-allowed Ruby to 2.2+ 2.3
• RuboCop config: Explicitly set the chosen Ruby requirement
• CI: Drop Ruby 2.1, 2.2 - See Proposal: reduce size of CI matrix #90
• CI: Add JRuby 9.2.6.0 (latest GA release)

Then lint to the new rules.

[olleolleolle]

@sickill Do you agree with this Ruby version policy? (Same as Ruby-the-language's.)

[olleolleolle]

@sickill Would you want to keep more specific versions in the Gemfile?

[ku1ik]

@olleolleolle in general I believe that if there's no practical, immediate reason to drop support for older language runtime version then one should not drop it. Some people on EOL-ed Ruby (many people are stuck) could benefit from bugfixes and new features if the new gem version doesn't require newer runtime. Frankly, I'm not using Ruby anymore (for few years already) so I'm not sure how many people use older releases, and how many of them are EOL-ed 😬

Having said that, it would definitely be simpler to stick to the same rules as Ruby does, not requiring a case by case decision from us now and then. So let's do this :)

And definitely no need for more specific versions in Gemfile 👍

[ku1ik]

🏆 👌

[stanhu]

@olleolleolle Would you mind releasing a new version so dependencies? My Rubocop is failing due to this error:

$ bundle exec rubocop
Error: RuboCop found unsupported Ruby version 2.1 in `TargetRubyVersion` parameter (in vendor/ruby/2.6.0/gems/rainbow-3.0.0/.rubocop.yml). 2.1-compatible analysis was dropped after version 0.58.
Supported versions: 2.3, 2.4, 2.5, 2.6, 2.7
ERROR: Job failed: exit code 1