ffmpeg: add new branch 2.6.1 (no switching yet)

/cc maintainers @codyopel, @fuuzetsu.

Release notes don't indicate any new options or dependencies.
http://git.videolan.org/?p=ffmpeg.git;a=blob;f=RELEASE_NOTES;hb=release/2.6
APIchanges looks like we should be able to replace 2.5 by 2.6
without anything breaking (belongs to staging and needs some testing).