replace pubSub()with pubSubLoop() in redis example

as per this change in predis:

https://github.com/nrk/predis/commit/8533dbdb0b3e3ee02d0eed474a09dcd659afb267

> Client::pubSub() still works like usual by returning a new pub/sub
> context, but it is now considered an alias of Client::pubSubLoop().

> This change is necessary in preparation for the next major version
> of Predis where Client::pubSub() will be used for the new PUBSUB
> command introduced in Redis 2.8.