Changing last_ip_check to allow single IPv6

Previously, last_ip_check would throw a 403 Forbidden when trying to remove
the last IPv4 address. Embracing the future, we now allow a single IPv6 to
be assigned to ports as well. RM10055
Merge pull request #25 from insequent/add_ipv6_to_last_ip_check

Changing last_ip_check to allow single IPv6