Fix the build

Here are some fixes to bring back the beloved green build :green_heart:.

- Before: https://travis-ci.org/colszowka/simplecov/builds/4747117
- After: https://travis-ci.org/infertux/simplecov/builds/4754150

Please look at individual commit messages for more info.

I haven't bothered to fix it for _rbx-19mode_ since it's not officially supported (which is cool because there's a lot of failures :P).