Promote ffmpeg 2.x to stable

We've been keeping ffmpeg 2.x as the "devel" version for awhile now due to compatibility issues. I've gone through core and tested everything that uses ffmpeg. I found a few incompatibilities, but in all cases those fixes have already been addressed upstream; I've backported small patches to fix them.

Given that everything in core (plus the taps I have tapped, e.g. opencv from homebrew/science) now builds with ffmpeg 2.x, I'd like to promote it to the default version.