Change Version Of phpredis Used On Travis

[danhunsaker]

Looks like phpredis is failing to build in the Travis tests, which gives false negatives to a lot of PRs... Specifically, #107, #96, #95, #93, and #94. We probably want to use a different version of phpredis where/until this gets fixed upstream.

[chrisboulton]

Looks like the phpredis folks have cut a new build that fixes the issue - I'll have a mess about over the weekend.

[chrisboulton]

Fixed in 3f5937a.