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.

.travis.yml

-dist: xenial
+dist: bionic  # ubuntu 18.04
 language: python
 
 python:
@@ -6,7 +6,7 @@ python:
   - "3.6"
   - "3.7"
 
-env: CUDA=9.2.148-1 CUDA_SHORT=9.2 UBUNTU_VERSION=ubuntu1604
+env: CUDA=10.1.105-1 CUDA_SHORT=10.1 UBUNTU_VERSION=ubuntu1804
 
 # Ref to CUDA installation in Travis: https://github.com/jeremad/cuda-travis
 before_install:
@@ -23,7 +23,7 @@ before_install:
   - PATH=${CUDA_HOME}/bin:${PATH}
 
 install:
-  - pip install Cython
+  - pip install Cython torch==1.2
   - pip install -r requirements.txt
   - pip install -r tests/requirements.txt