Bug 837631 - Upgrade Virtualenv to 1.8.4 and backport upstream fixes. r=glandium, a=bajaj

python/virtualenv from mozilla-central at feccfce43b59 replaces the
existing virtualenv.
Bug 865537 - Part 1: Add nsRange::SetEnableGravitationOnElementRemoval to suppress 'gravitation' behavior on node removal. r=smaug, a=bajaj
Bug 865537 - Part 2: The Range used to track the source element pointer for a media element should not gravitate outside the media element. r=cpearce, a=bajaj
Bug 867482. r=bhackett, a=bajaj