allow newer phpunit version for php 7.2

Author: Robbert Müller

composer.json

@@ -37,7 +37,7 @@
     "phpmd/phpmd": "^2.6.0",
     "phpmetrics/phpmetrics": "^2.3.2",
     "phpro/grumphp": ">=0.11.0,<1.0.0,!=0.11.4",
-    "phpunit/phpunit": "^5.7",
+    "phpunit/phpunit": "^5.7|^6.5",
     "robmorgan/phinx": "^0.9",
     "scheb/tombstone-analyzer": "^0.3",
     "sebastian/phpcpd": "^3.0",