HHVM: warm up the JIT by making web requests in Upstart post-start

Add an optional parameter, 'warmup_urls', that takes an array of URLs to
request from localhost on HHVM startup. The Upstart job configuration will
iterate on these in the job's post-start stanza.

Change-Id: I1643be9c00dbda4c037fc1b70061a4ed5da12228