AppVeyor CI failure: cannot retrieve FFmpeg dependency

[valnoel]

The FFmpeg builds for Windows URLs are no more available:

• http://ffmpeg.zeranoe.com/builds/win32/dev/ffmpeg-2.4.5-win32-dev.7z
• http://ffmpeg.zeranoe.com/builds/win32/shared/ffmpeg-2.4.5-win32-shared.7z

Solutions:

• find a new online repository with 2.4.5 version of FFmpeg built for Windows 32bits
• build FFmpeg-2.4.5 for Windows 32bits on our side and upload the dlls on a specific avTranscoder repository. On github for instance?
• adapt avTranscoder to the available 3.2 version of FFmpeg

[cchampet]

We should choose to support ffmpeg3 : see issue #252 :)
I updated the project settings: AppVeyor's build is no more a require when checking a PR.