Updated PHPUnit to 6.5 (stable)

Author: mrwogu

composer.json

@@ -44,7 +44,7 @@
         "justinrainbow/json-schema": "~5.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.0",
+        "phpunit/phpunit": "^6.5",
         "sami/sami": "dev-master",
         "psr/log": "~1.0",
         "satooshi/php-coveralls": "~1.0",