Enable Redis service in Travis configuration

Our tests started failing with:

    Error connecting to Redis on 127.0.0.1:6379 (ECONNREFUSED)

Investigation reveals we now need to explicitly enable the Redis server
by including it in the services list.

Change-Id: I4c3b752212383b0629e3c34e51cc28b163e19c2f
Reviewed-on: https://gerrit.causes.com/33181
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>