use the correct args when checking for required binaries

[mliang2]

The current dependencies checker runs the ffmpeg, ffprobe, and exifinfo commands on each loop and doesn't suppress the output. That results in the command output being sent to the browser.

The current ffmpeg check is broken b/c it runs ffmpeg 1>&2 /dev/null, which doesn't make any sense. That command returns a exit code 127 b/c it's telling ffmpeg to read from /dev/null.

This PR removes the incorrect checks and also use the proper command args to verify the command exists.

[EddyLB]

The variable $binary contains the path and the loop cannot identify the args from the defined keys. I close this PR and will soon commit a fix but thanks for raising the issue.