Remove lowest phpunit version

Author: alcaeus

composer.json

@@ -26,7 +26,7 @@
         "doctrine/migrations": "^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7|^6.4|^7.0",
+        "phpunit/phpunit": "^6.4|^7.0",
         "mikey179/vfsstream": "^1.6",
         "doctrine/coding-standard": "^5.0",
         "phpstan/phpstan": "^0.9.2",