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.
@dcelasun that's actually incorrect, you can still use plain `EVAL` and `EVALSHA` commands like any other client library, in fact the script command abstraction in Predis is built __upon__ these commands and does not replace them in any way.