Drop Ruby 1.9.3 support, add testing on 2.3/2.4

[Evesy]

Pull Request Checklist

RE https://github.com/sensu-plugins/sensu-plugins-feature-requests/issues/27

General

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

• [x] RuboCop passes

• [x] Existing tests pass

Purpose

Remove support for Ruby 1.9.3

Add testing on 2.3.0 & 2.4.1

Known Compatablity Issues

Breaking change for 1.9.3 users.

[majormoses]

@Evesy you missed calling it out in the breaking changes, I managed to catch that before release so it's all good.

[majormoses]

released: https://rubygems.org/gems/sensu-plugins-php-fpm/versions/1.0.1