Could this be solved by using a different PHPUnit version depending on
the branch? I followed a link in
https://github.com/travis-ci/travis-ci/issues/406 which leads to a dirty ;
but viable solution which just downloads an older version and executes
that:
https://github.com/OpenACalendar/OpenACalendar-Web-Core/commit/84abf236c14c4abbd36a95790449d5dee59db12c
Could not open input file: ../phpunit-5.7.phar