Remove ":" popping from redis auth strings.

Because Iris Couch requires an AUTH command in the format your_hostname.redis.irstack.com:your_password, we need to not strip the part before the colon in that case. Since we don't really need it in any case (just don't pass the username: in there), it's simplest to not alter the auth string.

https://github.com/josephg/ShareJS/pull/162