Periodically build notebooks to ensure they haven't broken

APIs and data change, and over time our notebooks end up with errors. #42 is one example of this.

Let's add a CI layer that runs the notebooks periodically, so we can be notified when things break, rather than finding out at or soon before a tutorial when we're trying to teach them....

This may require some creative thinking to get the tokens working correctly.

Several great suggestions how to get the notebooks running in Travis or GitLab here: https://twitter.com/yoyehudi/status/1143118846234173440