sensu-plugins/sensu-plugins-pushover

## Pull Request Checklist

**Is this in reference to an existing issue?**

#### General

- [ ] Update Changelog following the conventions laid out on [Keep A Changelog](http://keepachangelog.com/)

- [ ] Update README with any necessary configuration snippets

- [ ] Binstubs are created if needed

- [ ] RuboCop passes

- [ ] Existing tests pass

#### New Plugins

- [ ] Tests

- [ ] Add the plugin to the README

- [ ] Does it have a complete header as outlined [here](http://sensu-plugins.io/docs/developer_guidelines.html#coding-style)

#### Purpose

#### Known Compatability Issues