Libav has been broken due to the hwdec changes. This was always a
temporary situation (depended on pending patches to be merged), although
it took a bit longer. This also restores the travis config.
One code change is needed in vd_lavc.c, because it checks the AV_PIX_FMT
for videotoolbox (as opposed to the mpv format identifier), which is not
available in Libav. Add an ifdef; the affected code is for a deprecated
option anyway.