Update to minitest 5

Greetings, we're looking to fix some packages in Fedora the were broken by a recent update to minitest (now at 5.3.1).

compass was one of the gems that was broken due to it's dependency on the old API.

http://koji.fedoraproject.org/koji/packageinfo?packageID=10224

I went through and patched the source to work against the new API and am sending a PR for hopeful inclusion upstream.

Thank you for your time.