Update menu_css_names_menu_tree for PHP 5.5

The function menu_css_names_menu_tree (line 162 menu_css_names.module) uses preg_replace with the /e switch, which is deprecated as of PHP 5.5 (http://php.net/manual/en/migration55.deprecated.php).

Here is an updated bit of code using the preg_replace_callback function. I've tested it on a PHP 5.5 install, and on a PHP 5.3 install, without issues.

Old code, line 164:

$menu = preg_replace('/(<li\s+class="[^"]+)("><a\s+[^>]+>)(.+?)(<\/a>)/e', "'\\1 ' . _make_class_name('\\3') . '\\2\\3\\4'", $menu);

Replace that with this:

  $menu = preg_replace_callback(
    '/(<li\s+class="[^"]+)("><a\s+[^>]+>)(.+?)(<\/a>)/',
    function ($m) {
      return $m[1] . ' ' . _make_class_name($m[3]) . $m[2] . $m[3] . $m[4];
    },
    $menu
  );

I've spread the lines out for legibility for those encountering preg_replace_callback for the first time.

Figuring out how to patch via git is on my to-do list, so I'll try to roll a proper patch. But feel free to do so and move this little issue along.