HHVM: Set a fatal handler that logs traces

Follows change I5ff22ac23 in operations/mediawiki-config. This patch sets
'/srv/mediawiki/wmf-config/HHVMRequestInit.php' as HHVM's
request_init_document, which will cause it to run at the very beginning of each
request, before any other PHP code. The purpose of this file is to register a
handler for catchable fatal errors that logs the error + stack trace to syslog.
(We configure rsyslog to forward HHVM log messages to fluorine.)

Change-Id: Id93b9eaa177911a8e5dbb4d01541f57996831057
Merge "HHVM: Set a fatal handler that logs traces" into production