Run maintenance/install.php with PHP5

Running it with HHVM seems to require a lot of memory. The HHVM team has
indicated that optimizing the performance and resource utilization of
short-lived CLI scripts is not a priority (which is understandable),
so just run it with Zend PHP for now.

Bug: 67743
Change-Id: Ic4fd21c2d6d6afd46db8887f119ce17005bd78b8