Add code coverage and badge(s)

[casperboone]

This PR brings this repository up to date with recent changes in the skeleton. The primary part of this PR is a code coverage set up and badge (https://github.com/laravel-notification-channels/skeleton/pull/6).

Also, if this repository did not have the following already, I have:

• added PHP 7.1 to .travis.yml
• fixed a typo in the changelog