Add CI/tests

[ace-n]

Due to some recent Datastore updates, some GCP Next participants ran into errors with this lab during the event.

IMHO, we need some way to automatically/periodically test this and other codelabs - to make sure that doesn't happen in the future.

[tswast]

There are very few codelabs with Travis tests https://travis-ci.org/googlecodelabs so there shouldn't be an issue using that.