Integrate code coverage into Travis pipeline and GitHub PR workflow

This is a follow-up to #6875. Related to https://github.com/magefree/mage/commit/d481172eb4ce193e84156a88339dfdddb2865af9.

Now that coverage exists, we need to upload the results to a service that will track the coverage over time and that integrates with GitHub to share the progress in the pull requests.

The 2 I most commonly come across are:

* https://coveralls.io/
* https://about.codecov.io

Both have integrations and examples showing how to integrate into your current Travis workflow. Here are links to the related maven plugins:

* [Coveralls plugin](https://github.com/trautonen/coveralls-maven-plugin) / [example PR output](https://github.com/sinonjs/sinon/pull/2010#issuecomment-486888728).

* [CodeCov plugin](https://github.com/codecov/example-java) for Jacoco / [example PR output](https://github.com/sinonjs/sinon/pull/2382#issuecomment-858286228)

I believe the first step is to setup an account with one of the selected services. I can do the work of adding the plugins and such, but an admin would need to setup the project in Coveralls or CodeCov and supply any API keys or related artifacts.