Install CUDA toolkit on Travis

.travis.yml

@@ -7,7 +7,7 @@ language: python
 virtualenv:
   system_site_packages: false
 before_install:
-  - sudo apt-get install -y ninja-build build-essential
+  - sudo apt-get install -y ninja-build build-essential nvidia-cuda-toolkit
 addons:
   apt:
     update: true