Fix version for PHP Mess detector

Author: chevli

composer.json

@@ -16,7 +16,7 @@
     "magento/zendframework1": "~1.12.16"
   },
   "require-dev": {
-    "phpmd/phpmd": "@stable",
+    "phpmd/phpmd": "^2.6",
     "squizlabs/php_codesniffer": "1.5.3",
     "sebastian/phpcpd": "2.0.0",
     "satooshi/php-coveralls": "^1.0",