Assign Varnish cache IPv6 to Ysul

Summary:
There is a DNS record agora.nasqueron.org AAAA 2001:470:1f13:9e1::cac:7e:1.

This matches a Varnish cache for the wiki.

Ref T1243

Test Plan: ping agora.nasqueron.org (in IPv6)

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1243

Differential Revision: https://devcentral.nasqueron.org/D1268