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.
The issues I see with this approach:
- it adds overhead in case someone would like to use plain strings
- it makes impossible to use a custom serializer (igbinary, msgpack, json_encode etc.)

That is why I believe the decision should be made on a higher level. And the serialization functionality can be easily implemented via a simple decorator, like this one: https://github.com/rybakit/phive-task-queue/blob/master/src/SerializerAwareQueue.php.