[STANDBY] icanhazip: use ipv4 subdomain to receive only ipv4 addresses

If merged, this should finally bring online the ipv6-capable 12% of the network which is currently isolated/broken.

However this PR will be superseded by whatever comes out of #614 

This PR would effectively switch off ipv6 and I don't like doing that unless we really have to, so I'm not recommending to merge it now if ever. It does however do the job if necessary.