guard-spork fix

Without this fix, guard-minitest + spork  is totally non-working, as far as I can tell.

Both https://github.com/guard/guard-minitest/pull/42 and https://github.com/guard/guard-minitest/pull/41are along the right lines, but this version gets rid of all `-r` lines *and* the `test_helper.rb`.  Note that Ryan Davis himself [confirmed](https://github.com/guard/guard-minitest/pull/42#issuecomment-11110533) that all you should need is a `foo_test.rb` file, and that's good, because `spork-minitest` doesn't work with anything more than that.

Thank you.