Update docs for running tests on travis #16260

We need to update the docs in https://developer.matomo.org/guides/tests-travis to mention bionic distribution in the command for 4.x-dev (which I think is not used by default) and maybe in https://developer.matomo.org/guides/migrate-matomo-3-to-4 we could also mention they might need to run the generate travis yml command to run tests against 7.2 and 7.4?

I basically mean eg --php-versions="7.2,7.4" --distribution="bionic" --sudo-false --verbose

created PR in https://github.com/matomo-org/developer-documentation/pull/374