Use the phpmd repo instead of the website

phpmd is currently orphaned and looking for a new maintainer. While it's
in this unknown-intermediate stage, we should probably point to the repo
anyway because the website is currently down and will probably stay that
way until a new maintainer is found.

README.md

 
 * [PHP CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer) - A library that detects PHP, CSS and JS coding standard violations.
 * [PHP CS Fixer](https://github.com/FriendsOfPHP/PHP-CS-Fixer) - A coding standards fixer library.
-* [PHP Mess Detector](https://phpmd.org/) - A library that scans code for bugs, sub-optimal code, unused parameters and more.
+* [PHP Mess Detector](https://github.com/phpmd/phpmd) - A library that scans code for bugs, sub-optimal code, unused parameters and more.
 * [PHPCheckstyle](https://github.com/PHPCheckstyle/phpcheckstyle) - A tool to help adhere to certain coding conventions.
 * [PHPCPD](https://github.com/sebastianbergmann/phpcpd) - A library that detects copied and pasted code.