Your settings.php file must be modified to enable Authcache ($conf['cache_backends']). See README.tx

I get this message

'Your settings.php file must be modified to enable Authcache ($conf['cache_backends']). See README.tx'

when i go to

'admin/config/development/performance/authcache'

my settings are

$conf['memcache_servers']  = array('localhost:11211' => 'default');
$conf['cache_backends'][] = './sites/all/modules/memcache/memcache.inc';
$conf['cache_backends'][] = './sites/all/modules/authcache/authcache.inc';
$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';

what i'm missing?

Thank you