Some distributions use ffmpeg instead of avconv

[dreieinhalb]

Some distributions, in my test case Arch Linux, use (still?) ffmpeg instead of avconv. So the script breaks because of the buildin test, if avconv is installed.

[dreieinhalb]

Possible solutions are:

• buildin something like this in the script : avconv is not available on Arch Linux's official repos wargio/Twitch-Streamer-Linux#2 (comment) (clean solution)
• do a simple symlink: ln -s /usr/bin/ffmpeg /usr/bin/avconv (workaround solution)