Limit results to published pages

Use the public() method of PageManager to filter published pages rather than direct attribute access.

When I upgraded to Django-CMS 3, my HTMLSitemap was mysteriously blank. This call to public() remedied the problem. The public method existed in previous version of Django-CMS. Judging by the use in the Django-CMS code, it appears to be the officially sanctioned method of retrieving published pages.