Minitest rails compat

This pull requests gets `minitest-rails-shoulda` compatible with `minitest-rails 0.3.0` only. 

To get us to > 0.5.0 compat some more work will need to be done with the tests. (They are currently pretty wound up with how `minitest-rails` was implemented.)

If you would like, I can update the README and CHANGELOG as part of this commit. I don't usually like to do that as part of an external PR but it seems to make sense that we release a quick version after this update so we can then go straight into 0.5 compat (which we need for `thincloud-test-rails`.

Not to mention that the README is still reading the updated message from the original repo.