Good point. Although our guidance is certainly the safest option, I think the practice outlined in the Thoughtbot article is sound, i.e. pessimistic versions for gems you know (and trust to) follow semantic versioning; exact versions for those that don't. It makes keeping gems up to date a lot easier (e.g. `bundle update` will bump all patch versions on pessimistically locked gems). I can get a pull request into the style guide if others agree?