reprepro: import Facebook's HHVM pkgs, for Labs / Vagrant usage only

The current set of packages provided by Facebook have a number of issues that
Faidon (and others) have documented in Debian bug report #727085
(<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=727085>). There appears to
be some momentum in the direction of repackaging HHVM and doing a proper job so
that it can be included in Debian. For this reason, these packages are not to
be installed on production hosts. They are included simply to make testing on
Labs and MediaWiki-Vagrant easier. The author of this commit vows not to use
the presence of these packages in reprepo as a "fact on the ground" to drive
production deployment later.

Change-Id: I6c57d34105af5bf60dcd407621ded10a364fd40d