Search form support for WordPress 3.6+

Fix per #772.

For backwards compatibility you can use output buffering:
````
function roots_get_search_form($form) {
  ob_start();
  locate_template('/templates/searchform.php', true, false);
  $form = ob_get_clean();
  return $form;
}
add_filter('get_search_form', 'roots_get_search_form');
````