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",