Summary:
This PR fix the issue vis_one_image_opencv raises ValueError.

>Since OpenCV 3.2, findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.
https://docs.opencv.org/3.3.1/d4/d73/tutorial_py_contours_begin.html

and this line also assumes opencv>=3.2
https://github.com/facebookresearch/Detectron/blob/master/lib/utils/vis.py#L326

so, It may be better to set OpenCV version specification to 3.2 or higher in Installation guide.
Closes https://github.com/facebookresearch/Detectron/pull/9

Reviewed By: rbgirshick

Differential Revision: D7009465

Pulled By: ir413

fbshipit-source-id: 3b2cea6c94da1de93927a6d11724f104c063dca1