upgrade phpunit to 6

composer.json

@@ -17,7 +17,7 @@
         "psr/log": "^1.0.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": "^6.5",
         "graylog2/gelf-php": "^1.4.2",
         "sentry/sentry": "^0.13",
         "ruflin/elastica": ">=0.90 <3.0",