Use system ruby on OS X, update bundle

Hey @stephsharp could you test this out. Instead of relying on Ruby 1.9.3, this now just uses system ruby on OS X (i.e, 2.0.0). You should be able to check out this branch and just:

```
$ bundle install
$ dashing start
```

...and if it works ok we should be able to merge it (I think). Not sure what Heroku will think about us removing the version constraint information, but if it works on both our machines I think we can cross that bridge when we come to it.