Wercker box version update to run RethinkDB 1.13

I've just [pull requested](https://github.com/mies/box-rethinkdb/pull/1) some changes to the wercker box used for the testing of **gorethink**.

So, as soon as @mies merges the commits it will be possible to run RethinkDB v1.13 and tests on #98 should start passing.