Enable Cluster IPv6 Support

So... apparently nobody has ever tried to start a cluster with IPv6 addresses?

This PR is just two simple fixes to allow IPv6+Redis Cluster.

:iphone: 
Can we keep this more idiomatic Ruby?

```
if s.length < 2 # no then, no 'not'
   ...
end
port = s.pop # parentheses are omitted when no arguments given
```