Allow lower phpunit version for php7.2

Signed-off-by: Andreas Frömer <andreas.froemer@check24.de>

Author: Andreas Frömer

composer.json

@@ -7,7 +7,7 @@
         "symfony/process": "^4.4 || ^5.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.0",
+        "phpunit/phpunit": "^8.5 || ^9.0",
         "squizlabs/php_codesniffer": "^3.5",
         "codeclimate/php-test-reporter": "dev-master",
         "slevomat/coding-standard": "^6.1",