Fix Symfony's it's not safe to rely on system timezone setting errors

Summary: Symfony requires a date.timezone setting in the standard php.ini
We can't do that so this modification adds a call to date_default_timezone_set
Closes https://github.com/facebook/hhvm/pull/4251

Reviewed By: @fredemmott

Differential Revision: D1683075

Signature: t1:1683075:1416091145:b2eca865d5811e41aa0669519f59ac8dc5188341