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.
Uhm... yes? There *is* one blank lane after the namespace declaration and there *is* a blank line after use statements. PSR-2 doesn't say anything about `namespace` being in the same line with `<?php`. But if it bothers you I can add a line between them.