Update HHVM for Wikimedia's build

* De-list Facebook's HHVM repo; we'll use apt.wikimedia.org now.
* Install hhvm-dev rather than hhvm-nightly
* Update file locations in Upstart script
* Get rid of static file hack.
* Make apt::repo support ensure => absent (as the docs false declare it already does).

Change-Id: I6ebc98816a369bdcd2161e7734f4e0aba24a7feb
Merge "Update HHVM for Wikimedia's build"