Update library/Centurion/Contrib/core/views/helpers/Navigation.php

In the helper "Centurion_View_Helper_Navigation" (inherits from Zend_View_Helper_Navigation) to use Centurion Navigations Helpers instead of Zend Navigations Helpers : zend helpers was also not registered in the plugin loader, so Developpers can not use Zend Navigation Helpers (like Sitemap).

This patch records Zend Helpers before Centurion Helpers to use them when we have found nothing in Centurion.