use circleci instead of travis?

[rabernat]

Repo2docker recommends using circleci for continuous building:
https://repo2docker.readthedocs.io/en/latest/howto/deploy.html

This is what I set up on pangeo_ocean_examples:
https://github.com/pangeo-data/pangeo_ocean_examples/blob/master/.circleci/config.yml

There are some advantages to doing it that way, including a fancy caching layer between PR and deploy steps that avoid rebuilding the image unnecessarily.

[jhamman]

Happy to go this way if we want to take advantage of the caching in circleci. I originally went with Travis because it was the shortest path to a working system but if someone wants to revisit that decision, I'm happy to help make that possible.

[yuvipanda]

A good reason to move to CircleCI now is that Travis has been taken over by a private equity firm, and is hence on its deathbed (for OSS projects, I would say). Move when we have the time!

[jhamman]

We're starting to get build timeouts due to the serial build config we currently have. I think now is a good time to revisit this issue.

pinging @ocefpaf and @rsignell-usgs.