Revert back ffmpeg version from 3.3.9 to 3.3.6 for AppImage, as sound do not...

Revert back ffmpeg version from 3.3.9 to 3.3.6 for AppImage, as sound do not work while playing video.
Now QtAV is upgraded to 1.12.0 to 1.13.0 (just released)
CCBUGS: 406428
CCBUGS: 399305
CCBUGS: 395200