This lets you use both Coveralls and the SimpleCov html formatter that you have already been using. If you decide that you only want to use Coveralls, then this can be updated and the `coverage` folder can be removed (you can also update the README to remove the image with test coverage info).

Conversely, if you don't want coveralls, then this can be reverted to what it was and the gem should be removed along with the `.coveralls.yml` file.