Allow RedisConnectionPool to be used with HHVM's Redis library

As of commit 2811e9d, HHVM includes a pure-PHP Redis client library with
method-for-method compatibility with phpredis: <http://git.io/ylDAyQ>.
RedisConnectionPool appears to work well with it save for the
extension_loaded() check. Practically a freebie!

Bug: 54158
Change-Id: I3ac12293a51a45e8f6a61a35bff7a3c063e16964