Revert "Add Gemfile.lock"

This reverts commit 611fa02f0353b7680a1d9bd67c270ae352a3deab.

I did a mistake by adding the Gemfile.lock. Gems shall not have a Gemfile.lock in their repository.
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/