Update Router.php

After upgrading PHP 5.3 to 5.4 this change was the only way to avoid memory leaks caused by preg_replace_callback() creating anonymous functions. I've tested different ways of using function preg_replace_callback() with real and anonymous functions in simple example files on my server and that's the only working result. I didn't find another solution. Tried to increase pcre.backtrack_limit and pcre.recursion_limit but no effect.