Travis: Fix tests (jsonschema!=3.0.0a1) and update for jupyterhub 9.0.1/python3.7

[rkdarst]

No description provided.

[rkdarst]

Is something going wrong with outside software? It seems that all tests are failing in all PRs, and many of them seem unrelated to the slurmspawner changes...

[rkdarst]

This now has a fix for the breaking tests (at least pip install jsonschema==2.6.0 worked except for a timeout, now I changed to jsonschema!=3.0.0a1).

Also, python3.7 his not yet fully supported by travis - you need to specify dist=xenial which I didn't do globally yet. So it now just adds 3.7-dev to the matrix and we can fix up later.

Assuming this works, please merge soon so we can get working tests again! Also if someone can restart the test runner on existing PRs, that would be great.

[rkdarst]

All green! Let's just hope that by jsonschema==3.0.0a2 it gets fixed upstream.

The previous run with jsonschema==2.6.0 had one failure because of a timeout, btw. Otherwise green too.