Resolve test machine failure (#460)

This is due to the latest PyTorch (version 1.3) from conda is built on
CUDA 10.1 while the version on the test machine is CUDA 10.0.

environment.yml

@@ -18,7 +18,7 @@ channels:
 dependencies:
 - python==3.6.8
 - pytorch==1.2.0
-- torchvision>=0.3.0
+- torchvision==0.4.0
 - fastai==1.0.57
 - ipykernel>=4.6.1
 - jupyter>=1.0.0