Substitute /dev/ instead of translating it

[nmollerup]

Pull Request Checklist

Is this in reference to an existing issue? https://github.com/sensu-plugins/sensu-plugins-disk-checks/issues/105

General

• [x] Update Changelog following the conventions laid out here

• [x] Update README with any necessary configuration snippets

• [x] Binstubs are created if needed

• [x] RuboCop passes

• [x] Existing tests pass

New Plugins

• [x] Tests

• [x] Add the plugin to the README

• [x] Does it have a complete header as outlined here

Purpose

Known Compatibility Issues

[majormoses]

Thanks for your contribution to Sensu plugins! Without people like you submitting PRs we couldn't run the project. I will review it shortly.

Apologies on the long delay for review.

[majormoses]

I have cut a release but our CI (travis is backed up) I will check back in a few hours and if its not released I will manually publish to rubygems.

[majormoses]

released: https://rubygems.org/gems/sensu-plugins-disk-checks/versions/3.1.1

[majormoses]

Great work