ffmpeg: bump to 2.3.1
[depends/gas-preprocessor] - bump gas to https://git.libav.org/?p=gas-preprocessor.git;a=commit;h=03e23216b7e657fdf202befc129ca94cd582c253 - needed for ffmpeg2.3 compilation on ios
ffmpeg: adjust API requirements for external ffmpeg to 2.3.1
Merge pull request #5047 from FernetMenta/ffmpeg

ffmpeg: bump to 2.3.1