mediawiki::hhvm: warm up the JIT in an Upstart task

Provision an Upstart task (a short-running process) that runs
when HHVM is started and that warms up the JIT by repeatedly
requesting URLs read from /etc/hhvm/warmup.urls.

Change-Id: Ie402e1318082e913a3ab5aff01737ee1162a3835