Migrate TravisCI to the GitHub app

[kumar303]

Is this a feature request or a bug?

neither, just a maintenance chore

What is the current behavior?

TravisCI is hooked up as a service integration, which is deprecated.

What is the expected or desired behavior?

We need to move to the TravisCI GitHub app, which may require opening a bug for access. See https://bugzilla.mozilla.org/show_bug.cgi?id=1486771#c9

[willdurand]

Note that, in theory, Travis is supposed to perform the service integration migration (which is, I believe, done already).

But then, there is the .org to .com migration, and this is not clear whether Travis will do this migration and/or when it will happen. So far, we can opt in for a beta migration: https://docs.travis-ci.com/user/migrate/open-source-on-travis-ci-com/#existing-open-source-repositories-on-travis-ciorg.