Remove and ignore Gemfile.lock

The gem itself doesn't need one, and test applications need to be bundled before tests can be run anyways.

Keeping a `Gemfile.lock` in the application directories means that the files will be updated whenever you run tests, and receive the version of the previous release if you're preparing to release a new version. No point in doing that. :)

After removing all `Gemfile.lock`s, running `rake tests:bundle tests:run` still works and is all green. :green_heart: