Update PHPMD

composer.json

@@ -64,7 +64,7 @@
         "friendsofphp/php-cs-fixer": "1.11.6",
         "phpdocumentor/phpdocumentor": "2.9.0",
         "phploc/phploc": "3.0.1",
-        "phpmd/phpmd": "2.4.2",
+        "phpmd/phpmd": "2.6.0",
         "phpunit/phpunit": "4.8.27",
         "sebastian/phpcpd": "2.0.0",
         "squizlabs/php_codesniffer": "2.7.1"