Create quibble-stretch-hhvm (replacing quibble-jessie-hhvm)

The 'hhvm' variant of Jenkins jobs for MediaWiki were still on
Debian Jessie, which does not match what we run in production,
and also means the Chromium version is EOL/unmaintained, stuck
as version 57 (latest version from Jessie: 71).

Change-Id: I83f81c35495486b8faa498351949bce5faefc6ed