The PPA suggested in docs (for installing ffmpeg) no longer exists

[GitTom]

For installing ffmpeg, the readme says "(For Ubuntu, the ppa:jon-severinsson/ffmpeg PPA provides recent builds)", but that PPA no longer exists.

Installing ffmpeg on Ubuntu is actually pretty messy. The full story and possible solutions are recounted here:

http://askubuntu.com/questions/432542/is-ffmpeg-missing-from-the-official-repositories-in-14-04

Perhaps it would be best to just link to that stack question.

[njoyard]

Ffmpeg gets back to ubuntu anyway. I think we'll get rid of this paragraph altogether.