Downgrade composer phpunit dependencies

Author: TysonAndre

composer.json

@@ -6,7 +6,7 @@
         "nikic/PHP-Parser": "3.0.5"
     },
     "require-dev": {
-        "phpunit/phpunit": "~6.2"
+        "phpunit/phpunit": "^5.7"
     },
     "suggest": {
         "ext-ast": "~0.1.4"