Non-free algorithms like SIFT and SURF causing tests to fail.

[RobRoseKnows]

See opencv/opencv-python#126. As of OpenCV 3.4.3, the patented algorithms such as SIFT and SURF are hidden behind the OPENCV_ENABLE_NONFREE=1 flag. They are no longer included in the Python bindings which is causing the tests to fail on Travis.

Possible options are:

• Remove SIFT and SURF from the tests
• Add try-except logic in kp_method initialization to raise a warning when SIFT and SURF are used.

[AdamSpannbauer]

Thanks for the catch, I had forgotten to turn on a recurring Travis build (turned on now).

Fix to come.

[AdamSpannbauer]

SIFT and SURF dropped from tests and a note about them has been added to the docs

thanks again for the catch @RobRoseKnows