Use of the official repository with the latest version of RethinkDB

Good day.

A new version of RethinkDB has recently been released. So the ``wercker-box.yml`` file has to be updated.
And as ``ppa`` is out of date it is probably a good idea to replace it by the [RethinkDB official repository for Ubuntu](http://rethinkdb.com/docs/install/ubuntu/).

So, please, accept the pull request.