travis: Fix sending coverage (use PHP 7.4)
travis: Fix sending coverage (use PHP 7.4)

Per https://app.travis-ci.com/github/wmde/Diff/jobs/584745632
"This version of PHPUnit does not support code coverage on
PHP 8", thus use the PHP 7.4 jobs to generate coverage.

Also set, the apparently needed, XDEBUG_MODE=coverage.

wmde/Diff