require 'minitest' for people who need 'minitest/test' directly.

hey ryan,

this fixes an issue where requiring minitest/test directly does not require minitest, so it blows up depending on runnable.

I'm not 100% sure if this is how you want the library to be used, but I figured it was worth sending along the pull request. Currently porting to minitest 5 so I'm running into these kinds of issues.

Thanks again for a great library.

-Erik