Fix travis main tests

[aktech]

This fixes the broken travis tests (for env MAIN-TEST=true). This a step towards getting #408 merged.

• Add worker certs config for cluster_and_security fixture
• Install latest node to fix jupyterhub tests (unable to start proxy server without it)
• shutdown cluster in the test_client_reprs test causing domino effect on the following tests and making them flaky
• Make black happy
• Run travis on all branches

✅ Passing tests can be seen here: https://www.travis-ci.com/github/aktech/dask-gateway/builds/233513175

@TomAugspurger @martindurant @jacobtomlinson @douglasdavis

[martindurant]

This looks certainly an improvement, pushing it in.

[aktech]

@martindurant Do you know why travis is not running? did we ran out of free credits?

[martindurant]

I don't know. I don't manage travis on this project, I only came in to try to help recently.

[consideRatio]

Travis is dead for open source is my understanding =\ they cut down on their free quota by a lot and did it without a heads up.

In the jupyterhub org we systematically used travis, but have now migrated to gh actions