dnspoison module: IPv6 addresses cause false positives

https://github.com/NullHypothesis/exitmap/issues/60:

When the exit relay returns IPv6 addresses for a tested domain the dnspoison module claims to have found a DNS poisoning exit, but this is a false positive since the module does not support IPv6 and interprets the returned data as IPv4 address.

This is causing almost daily false positives.