Fix phpunit version

Author: ddrv

composer.json

@@ -17,7 +17,7 @@
   },
   "require-dev": {
     "nyholm/psr7": "^1.3",
-    "phpunit/phpunit": "^6.5",
+    "phpunit/phpunit": ">=6.5",
     "squizlabs/php_codesniffer": "^3.5",
     "webclient/fake-http-client": "^1.0"
   },