ffmpeg on Ubuntu 14.04 does not leverage postproc

I first reported this at https://groups.google.com/forum/#!topic/islandora/bqyeJg5heT4. 

Ubuntu 14 LTS ships with ffmpeg as avconv and without a library that seems essential, libpostproc. Without it, the hardcoded flag -movflags faststart fails. I can't find in the ffmpeg documentation where this library is needed, but it does state that -movflags is a post process operation. 

A second bug I found when debugging this is that the temporary files are not deleted upon failure. If you upload a new video to create a new mp4 datastream, it will fail as the file already exists. The variable $out_file on line 28 of derivative.inc could perhaps append a random number so that failed file writes would not stop that object from having a successful derivative made until /tmp is cleaned.