Add CUDA continuous integration
It would be great to have CI for the CUDA code.

While it's unrealistic to expect to run tests on GPU, it should be possible to at least install the CUDA toolkit and compile against it.

Issues/repo to follow:
- Travis (compile build): https://github.com/travis-ci/apt-package-whitelist/issues/587
- Travis (runtime test): https://github.com/travis-ci/travis-ci/issues/5911
- Appveyor (build script): https://github.com/willyd/appveyor-cuda-test
- Appveyor (runtime test): https://github.com/appveyor/ci/issues/342 