Switch MW-Vagrant TMH to use ffmpeg instead of avconv
Switch MW-Vagrant TMH to use ffmpeg instead of avconv

Works around horrible bugs in avconv on Ubuntu 15.04, and
lets us consume and produce VP9 and Opus now that we have
an updated ffmpeg.

Bug: T106612
Change-Id: I247d13c7018a72d881dde7628644e4758739c35e

stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,HarriAlasi/wiki3dviewer,stuartbman/mediawiki-vagrant,HarriAlasi/wiki3dviewer,wikimedia/mediawiki-vagrant,HarriAlasi/wiki3dviewer,HarriAlasi/wiki3dviewer,HarriAlasi/wiki3dviewer,HarriAlasi/wiki3dviewer,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,HarriAlasi/wiki3dviewer,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant