Update CI to use PyTorch nightly

Author: fmassa

.travis.yml

@@ -25,7 +25,7 @@ before_install:
   # Useful for debugging any issues with conda
   - conda info -a
 
-  - conda create -q -n test-environment python=$TRAVIS_PYTHON_VERSION pytorch scipy -c pytorch
+  - conda create -q -n test-environment python=$TRAVIS_PYTHON_VERSION pytorch-nightly scipy -c pytorch
   - source activate test-environment
   - |
     if [[ "$IMAGE_BACKEND" == "Pillow-SIMD" ]]; then