HHVM: Update the extension dir path for 3.3 changes

As of 3.3+, the extension directory is located in
/usr/lib/x86_64-linux-gnu/hhvm/extensions; previously it was in
/usr/lib/hphp/extensions. This patch updates the module for the new path
hierarchy. To prevent breakage on the production HHVM app servers (which have
not been upgraded yet), I copied over the extension directory from the
old path to the new one.

Change-Id: Id032f68a3b31f1339c798295f82f9e3d5d519bfe