Test Ruby 2.3.0, 2.4.1 & drop support of Ruby 1.9.3

[thomasriley]

Pull Request Checklist

sensu-plugins/sensu-plugins-feature-requests#27

General

• [x] Update Changelog following the conventions laid out on Keep A Changelog

• [x] RuboCop passes

• [x] Existing tests pass

Purpose

Add support for Ruby 2.3.0 Add support for Ruby 2.4.1

Known Compatablity Issues

Drop support for Ruby 1.9.3

[majormoses]

released: https://rubygems.org/gems/sensu-plugins-cgroups/versions/1.0.0