Use popen.communicate() to prevent subprocess hang

[tehKost]

Broken video files caused ffmpeg subprocess to hang

[coveralls]

Coverage decreased (-0.2%) to 56.962% when pulling a5c4823 on tehKost:master into f444f8c on Zulko:master.

[coveralls]

Coverage increased (+0.01%) to 67.459% when pulling 29c53d7 on tehKost:master into a1df46d on Zulko:master.

[mgaitan]

could we merge this?

[tburrows13]

It looks like this has already mostly been implemented elsewhere, however without the terminate().

[tburrows13]

Largely superseded by #926, but I'm going to merge this change which now adds proc.terminate(). I doubt that it is necessary, but I don't think that it can hurt.