Fixed running on python-opencv 2.4.8

When trying to run CMT on an opencv 2.4.8 installation, it complains
about missing parameters on cv2.calcOpticalFlowPyrLK. This was "fixed" by
passing None on both calls. I have no idea if it breaks other versions of
opencv/python-opencv

Signed-off-by: Luiz Carlos Cavalcanti <ext-luiz-carlos.cavalcanti@microsoft.com>