Locking phpunit version

Author: Kage

composer.json

@@ -45,6 +45,7 @@
         "twig/twig": "^1.0||^2.0"
     },
     "require-dev": {
+        "phpunit/phpunit": "6.5",
         "sensio/generator-bundle": "^3.0",
         "symfony/phpunit-bridge": "^3.0"
     },