phab: clone libphutil and arcanist from WMF repos
phab: clone libphutil and arcanist from WMF repos

Use the wmf/stable versions of libphutil and arcanist rather than the
upstream master.

Change-Id: I6b0ab5f3c01b69060ef5bd65e3b5ba5254852ce0

wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant