Set default timezone to UTC for PHP

php packages from Debian do not set this, nor does PHP upstream.
Lots of things depend on a default timezone existing and behave
poorly when they don't.

MediaWiki has long since worked around this but we can at least
give people a sane working environment.

UTC because there's other timezones? :)

Change-Id: I17c31fb59964915aa6848058c81d031e05fd48bc