Remove dependency on self to allow installation

This is a fix for the same issue as #11 - allows installation of the gem using newer rubygems. It isn't a good idea to use the Gemfile for dependencies, that's what the gemspec is for in gems. I tested this on ree 1.8.7, 1.9.3, 2.0 and rbx-2, and found that the test test_finding_empty_test_methods is failing. It was failing before this change, so I'm not sure what's up there.