Update PHP deps

Author: PowerKiKi

composer.json

@@ -28,9 +28,9 @@
         "webonyx/graphql-php": "^0.11.5"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5",
         "friendsofphp/php-cs-fixer": "^2.9",
-        "zendframework/zend-diactoros": "^1.7"
+        "zendframework/zend-diactoros": "^1.7",
+        "phpunit/phpunit": "@stable"
     },
     "license": "MIT"
 }