Use latest version of PHPUnit

Author: chadicus

composer.json

@@ -26,7 +26,7 @@
     },
     "require-dev": {
         "squizlabs/php_codesniffer": "~2.0",
-        "phpunit/phpunit": "~4.0",
+        "phpunit/phpunit": "^5.5",
         "satooshi/php-coveralls": "~0.6.1"
     },
     "autoload": {