Allow phpunit 4 for testing

Author: sirbrillig

composer.circleci.json

@@ -42,6 +42,6 @@
         "phpcsstandards/phpcsutils": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.0 || ^6.5 || ^7.0 || ^8.0"
+        "phpunit/phpunit": "^4.0 || ^5.0 || ^6.5 || ^7.0 || ^8.0"
     }
 }