Update composer.json

Author: ivopetkov

composer.json

@@ -19,7 +19,7 @@
         "ivopetkov/html5-dom-document-php": "2.*"
     },
     "require-dev": {
-        "phpunit/phpunit": "6.*",
+        "phpunit/phpunit": "6.*|7.*|8.*|9.*",
         "ivopetkov/docs-generator": "0.1.*"
     },
     "autoload": {