min. PhpUnit 8.0 to fix php7.4 warnings

Author: Mardl

composer.json

@@ -9,8 +9,8 @@
         "doctrine/dbal": "2.5.*",
         "pdepend/pdepend": "2.5.2",
         "phpmd/phpmd": "@stable",
-        "phpunit/phpunit": "~6.5.0",
-        "sebastian/phpcpd": "~3.0.0",
+        "phpunit/phpunit": "^6.5.0|^8.0.0",
+        "sebastian/phpcpd": "~3.0|~4.0|~5.0|~6.0",
         "squizlabs/php_codesniffer": "~3.4.0",
         "consolidation/robo": "~1.0",
         "mikey179/vfsstream": "~1.0",