Restore static file serving when using HHVM

Static files, most importantly images, were no longer served since
4975869.

To test try opening the following URL with and without the patch:
http://127.0.0.1:8080/w/skins/common/images/poweredby_mediawiki_88x31.png

Change-Id: I8afbab070cafa5b3d04cbb36ed01db46edbea170