Travis: build freeze PyTorch 1.2 (#1534)
Travis: build freeze PyTorch 1.2 (#1534)

* update Travis build

* freeze pytorch 1.2

* Install the latest pytorch instead of 1.2

* Revert "Install the latest pytorch instead of 1.2"

This reverts commit d269d2144f9c3b00816b00c31b2b7aa0c5c595f7.

open-mmlab/mmdetection,open-mmlab/mmdetection