CI: Flakiness for Saucelabs and Browserstacks

[devversion]

Sometimes we experience random build issues with Saucelabs or Browserstack on Travis CI.

I'm assuming that the problem is that we run too many builds concurrently.

Regardless of having the unlimited concurrency license, I think that the builds are affected by it.

We experienced similar things in M1, when we ran our tests of different modes.
After switching it back to one mode and running the tests sequential the builds ran fine.

Also we use the same license between M1 and M2, and who knows what other projects.

@jelbourn I recommend temporary disabling the optional builds (for testing) and also probably using a license only for M2 (then we can also show supported browsers banners)

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