[KNIFE-328] Remove the Gemfile.lock

http://tickets.opscode.com/browse/KNIFE-328

Closes #124

According to [Yehuda](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/), one should not commit the Gemfile.lock when a project is distributed as a gem.