Automation improvements:
- the docker images are built in Travis CI
- the code for the `nansat_base` image has been moved to the [docker-nansat-base](https://github.com/nansencenter/docker-nansat-base) repository
- the docker images are now available in the [nansencenter](https://hub.docker.com/search?q=nansencenter&type=image) organization in the Docker Hub