We are currently running tests against PHP 7.2 and 7.3 only. Actually it would be good to run tests against minimum and latest PHP stable release (7.2 and 7.4 at that point).

This won't be directly possible with our current travis setup as PHP 7.4 can't be used with trusty distribution (GD extension doesn't work there)

What actually needs to be done:

• Make travis scripts compatible with newer travis distributions (xenial / bionic) (see Allow using newer travis distributions travis-scripts#56)
• Update PHPUnit, as currently used version is not compatible with 7.4 (see Update to PHPUnit 8.5 #15581)
• Update core and plugin .yml files to use a newer distribution and run tests on 7.4