avformat: replace some odd 30-60 rates by higher less odd ones in  get_std_framerate()

Fixes Ticket4012

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 367c9d33d6dd1e8a85b63e14464e7e08ee1315cc)

Conflicts:

	libavformat/version.h

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>