use Hashie:Mash instead of ObjectifiedHash

Let's use `Hashie:Mash` instead of `ObjectifiedHash`. `ObjectifiedHash` doesn't work properly with ruby 1.8 as ruby 1.8 objects have `id` method. Yeah, I know that you don't test this gem with ruby 1.8 but I think it's ok to use `Hashie` gem here. There are only benefits from it (actually `ObjectifiedHash` breaks integration with [Errbit](https://github.com/errbit/errbit). And yes, we still support ruby 1.8 :( )