Bumping PHPUnit version requirement

composer.json

@@ -16,7 +16,7 @@
         "doctrine/common": "~2.4"
     },
     "require-dev": {
-        "phpunit/phpunit": "4.*",
+        "phpunit/phpunit": "^5.4.6",
         "symfony/console": "2.*"
     },
     "suggest": {