Skip IPv6 addresses in cname_lookup middleware.

It already skips IPv4 addresses, and since IPv6 is the future of the
Internet*, we should probably do the right thing with those too.

* IPv6: Just two years away for over fifteen years!

Change-Id: I54f1db4e936fd38d05ac8b5c709efba76525b9d2
Merge "Skip IPv6 addresses in cname_lookup middleware."