Remove Ruby 1.9.3 support; add Ruby 2.3.0 support

[cwjohnston]

Pull Request Checklist

General

• [x] Update Changelog following the conventions laid out on Keep A Changelog
• [ ] Update README with any necessary configuration snippets
• [ ] Binstubs are created if needed
• [x] RuboCop passes
• [x] Existing tests pass

  Purpose

This PR removes Ruby 1.9.3 support, and adds support for testing under Ruby 2.3.0, per the Sensu Plugins policy:

Ruby 1.9.3 (EOL): Linting is not done against it, and 1.9.3 will be supported where possible till Feb 2016 which is one year after EOL and 2 years after EOL was announced.

Known Compatibility Issues

Adds incompatibility with Ruby version < 2.0.0

[eheydrick]

We should remove codeship from the build since it's not used anymore.

[cwjohnston]

@eheydrick where's codeship in the build config? I'm not seeing it...

[eheydrick]

@cwjohnston in the github webhooks config