Add Predis support

As I've mentioned on Reddit a few days ago, this PR adds support for the Predis library.

Unit tests are passing, but this was a very quick attempt so let me know if there is anything missing and I'll amend the PR.

Thanks!

*Edit:* Travis fails since it can't locate Predis. Seems it can be fixed by updating travis.yml but I don't know how to do it.
My opinion on `eval` vs `multi/exec` is not final, but I will definitely not go with the implicit serialization for the reasons I mentioned earlier. It just goes against with the other queue implementations. Sorry.