Disable video feed in ffmpeg's flac conversion

Video feeds may be found in flac files, and the current ffmpeg
conversion command does not ignore them, causing beets to fail
to convert the file.

Add `-vn` to the command line to fix the problem.

Fix issue #712.
Disable the video stream in all ffmpeg conversions

Propagate the previous commit's change to all ffmpeg command lines.
Merge pull request #714 from brunal/master

Disable video feed in ffmpeg's flac conversion