Add remove_versions method

remove_versions maps to `/apps/:id/versions/delete` in the hockeyapp API

I also took the liberty of cleaning up some other aspects of the gem:
 * Removing `Gemfile.lock` (gems shouldn't have these, see [explanation](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/))
 * Marked testing (etc) dependencies as developer deps instead of runtime

I wasn't sure about adding `HockeyApp::App#remove_versions(options = {})` so I didn't bother -- `client.remove_versions(app, options)` seemed reasonable enough for my use case.

I was also going to bump the version but that seemed audacious ;)