Modified IPAddress::Prefix128 to allow /0 subnets

This fix is required similar to previous fixes for Prefix32. AIX systems have loopback address set to 
inet6 ::1%1/0 with prefix of 0. This is required for Chef ohai plugins.

references to old fix: 
https://github.com/bluemonk/ipaddress/commit/af56aabfcfa46723a2ee8e71a2e706bbd230ef62 
https://github.com/bluemonk/ipaddress/commit/120a5093a115c730bb6941285f32b5b20eeb6b93
https://groups.google.com/forum/#!msg/ruby-ipaddress/wElC8J8hSrk/oUSbq3sKQCMJ