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
Merge pull request #183 from bencevans/fix/travis

Fix Travis erroring due to Medibuntu shutdown
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.
Merge branch 'patch-1' of github.com:njoyard/node-fluent-ffmpeg into patch-1