updating ffmpeg to 3.1.x in rawhide

[Julian Sikorski]

Hi list,

now that the dust following the f24 release and related infra work
(thank you everyone involved) has settled, I believe we should update
ffmpeg to 3.1.x in f25. While we will likely need to update once again
to 3.2.x before f25 release (ffmpeg runs on 3 monthly schedule as
opposed to Fedora's 6 months), updating early allows to weed out any
build issues early on.
I have done a test rebuild and found the following:
- there are currently 40 packages dependent on ffmpeg
- out of those, 37 rebuilt correctly
- 3 have failed:
  - kodi: failure looks related to curl, not to ffmpeg
  - mlt: strange error, something about zval. build.log crashes gedit
  - xpra-codecs-freeworld: avcodec_decode_video2 is deprecated

Given the small number of failures, I would be aiming to push the new
ffmpeg to rawhide and rebuild dependent packages on Saturday. Please
speak up if you don't agree.

Best regards,
Julian