Make catbox-redis support unixsocket

In development we use a redis server running as a child process of node on a unixsocket, and node_redis connects to it over the socket. This small change would enable us to use hapi/catbox caching in our environment (doesn't work as is since catbox-redis expects the redis server to connect using host and port).

Its probably not that common, but this is a small non-invasive change. I have tested it, but I did not add any tests as I didn't want to require you to setup a redis server this way for your test suite.