Gemfile.lock should not be under version control

The `Gemfile.lock` should not be under version control when developing a gem. When the gem will be installed, only the constraints in the gemspec apply. It suggests a false environment when you version the `Gemfile.lock`.

You can find more about that topic in a [blog post by Yehuda](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/)