Fix writeToStream writing to stream after closing it

writeToStream closes the output stream when the ffmpeg process terminates, but 'data' events on the process stdout stream can still happen after that.

This patch fixes this problem by closing the stream on the stdout 'end' event.
Remove mediubuntu repo on travis build

As described on https://help.ubuntu.com/community/Medibuntu Medibuntu has shutdown, the packages moved to official Ubuntu archive
Update apt-get cache before install for Travis build