Remove output fps flag

Apparently, ffmpeg's x11grab device automatically handles
duplication without saying so in the output.

ffmpeg -f x11grab -framerate 60 [...] -vcodec libx264 out.avi
results in a 60fps file with 60*duration frames even if the input
is a lower framerate (IE: my system gets 40fps from x11grab, the
device will duplicate frames it misses to reach 60fps input, and
the output will naturally match the input at 60fps)
Shorter codec option format
Deprecated avconv, moved completely to ffmpeg