CI: Move linux builds from Azure to Travis?
I didn't check in detail, but I'd say that azure auto-cancel is now working (at least I didn't see any repeated PR, which the previous days happened frequently). But even with that, azure is significantly delayed compared to travis.

In #22854, I'm moving the docs build from travis to azure (takes more or less the same than a test suite). This will make things even less balanced (making azure be the bottleneck even more).

Not sure if I'm missing something, but would it make sense to move the 3 Linux jobs in azure to travis?

Also, I see in https://docs.microsoft.com/en-us/azure/devops/pipelines/licensing/concurrent-jobs-vsts?view=vsts#microsoft-hosted-cicd the next sentence: `Public project: Contact us to get your free tier limits increased.`. Does it make sense to check if that's an option?

CC: @TomAugspurger @jreback 