Untrack Gemfile.lock

A gem should be loser than an application wrt its dependencies. Don't
lock them to a specific version with the .lock.

See
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Remove lita 2.0 dependency restriction

This handler is compatible with Lita 3.0. No reason to not be
upgradable.