Support for Ruby 2.4 through 2.7

[mvz]

Summary

Update the set of tested and supported Rubies to 2.4 - 2.7

Details

• Bump minimum version in gemspec and rubocop config
• Stop building on 2.3 on Travis
• Start building on 2.7 on Travis
• Update rubocop and aruba dependencies, which also support Ruby 2.4+

Motivation and Context

This relates to #455 and lowers the maintenance burden.

How Has This Been Tested?

Travis will have to check on all supported Rubies.

Types of changes

Not sure.

• [ ] 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).

Checklist:

Not sure yet either.

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

[luke-hill]

@mvz can we move this PR into ready for review? As someone else has come forward to do some follow-up work to this.

[mvz]

@mvz can we move this PR into ready for review? As someone else has come forward to do some follow-up work to this.

Done! I'll also rebase this now that #451 has been merged.