Set PHPUnit version to the max current compatible

Author: kylekatarnls

composer.json

@@ -13,7 +13,7 @@
     "php": ">=5.3.0"
   },
   "require-dev": {
-    "phpunit/phpunit": ">=4.8",
+    "phpunit/phpunit": ">=4.8 <6.0",
     "codeclimate/php-test-reporter": "dev-master"
   },
   "minimum-stability": "stable",