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.
Well, phpredis client has a build-in serialization support, you don't need to serialize items yourself. And predis doesn't: https://github.com/nrk/predis/issues/29#issuecomment-1202624. If we leave this line then we have to put ✓** everywhere. I would just update this section with a note describing how to manage items with arbitrary type using a decorator.