Steven McCoy's IPv6 patches were merged into the master. The change
should be completely backward compatible. By default, IPv6 support is
switched off. To switch it on set ZMQ_IPV4ONLY option to false.
When you do so and if both peers support IPv6, the communication will
happen in IPv6.
If one of the peers doesn't support IPv6 the communication will
transparently fall back to IPv4.