Remove Gemfile.lock from repository

See:
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

> _**When developing a gem**, use the gemspec method in your Gemfile to avoid duplication. In general, a gem’s Gemfile should contain the Rubygems source and a single gemspec line. **Do not check your Gemfile.lock into version control**, since it enforces precision that does not exist in the gem command..._