Fix redis-trib.rb IP:Port disassembly for IPv6

IP format is now any of:
  - 127.0.0.1:6379
  - ::1:6379
Parse cluster state file in IPv6 compatible way

We need to pick the port based on the _last_ colon, not the first one.
Merge pull request #2096 from mattsta/cluster-ipv6

Enable Cluster IPv6 Support