avoid tracking Gemfile.lock fixes #248

Turns out the final problem with #248 was a bundler issue, solved by `bundle update`.

If you have a reason for including the lockfile in the repo I'd definitely be interested in learning more.

Otherwise, the `Gemfile.lock` [should not be tracked](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/).