Adjust to Homebrew's test environment changes

Homebrew has been changing their test infrastructure quite a bit.

The logs say they are switching to Minitest, and a refactor seems to be happening.

These changes are imperfect, but without them, Travis is going to start reporting
failures 100% of the time.
- minitest-colorize disabled, should find a way to re-enable it
- global `shutup` method defined, should use Homebrew's new version instead