Drop Ruby 2.2 support

[mvz]

Summary

Drop support for Rubies older than 2.3

Details

• Bump required Ruby version
• Bump RuboCop target ruby version
• Update RuboCop since latest version supports Ruby 2.3 and up
• Fix new RuboCop offenses
• Remove outdated version checks

Motivation and Context

This simplifies future maintenance by limiting set of supported Ruby versions. Dropping support for 2.3 before the final release is a distinct possibility. Projects that need support for older Rubies can stick with Aruby 0.x until they're ready.

How Has This Been Tested?

Travis will tell ...

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase without changing any existing functionality)
• Update documentation

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[mvz]

After this, I think we can release 1.0.0.alpha3.

[mvz]

I'm going to need to fix issues due to the frozen string comments.

[mvz]

@luke-hill I'm going to release alpha3 first.

[luke-hill]

Up to you if you want to do a quick-fix for the one versioned gem.

[luke-hill]

I've made the minor fixes @mvz just to try help you out. If CI passes I'll merge it in and crack on with some more auto-tidy stuff in the master branch.

[mvz]

Thanks @luke-hill. Merge at will! 😄