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.
I see your point, but I also see it as a trade off. The overhead is negligable (and I can make it only serialize/deserialize objects and arrays) and custom serializers are not too important (to me anyway).

Honestly, I'm not really willing to remove it as it would be useless to me without it. Since "being useful to me" (and hopefully to others) is the reason I submitted the PR in the first place, I'd rather use another library than implement my own serialization at every queue consumer.

Considering our differences of opinion on this, and use of `eval` instead of transactions, I'm willing to abandon this PR if you'd rather take a different approach. Thoughts?