Replace ffmpeg preset with "Main" profile

It seems the "hq" preset was removed from ffmpeg, so the encoding always
fails. Maybe someone wants to play with the encoding parameters some
more, but the input material isn't that good to begin with (at least on
the DVDs I have), so a bit more quality won't matter that much ;)
Merge pull request #73 from TobiX/fix-ffmpeg-call

Replace ffmpeg preset with "Main" profile