Make drupal purge varnish cache after node changes

local.settings.php

<?php

// Varnish

$conf['cache_backends'][] = 'sites/all/modules/contrib/varnish/varnish.cache.inc';
$conf['cache_class_cache_page'] = 'VarnishCache';
// Drupal 7 does not cache pages when we invoke hooks during bootstrap. This needs
// to be disabled.
$conf['page_cache_invoke_hooks'] = FALSE;
$conf['reverse_proxy'] = TRUE;
$conf['cache'] = 1;
$conf['reverse_proxy_header'] = 'HTTP_X_FORWARDED_FOR';
$conf['omit_vary_cookie'] = FALSE;