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.
`ExceptionalQueue` seems to be used only for tests. Are you sure you want `PredisQueue` to extend `ExceptionalQueue`? It would also be a lot slower then a direct `try/catch` block.