Authcache not enabled after adding ($conf['cache_inc']) to settings.php, per README.txt [#1220324]

Hello,
I'm attempting to configure AuthCache with Cache Router, and am receiving the "Your settings.php file must be modified to enable Authcache ($conf['cache_inc']). See README.txt." message after adding the following to my settings.php.

$conf['cache_inc'] = './sites/all/modules/authcache/authcache.inc';
$conf['cacherouter'] = array(
  'default' => array(
    'engine' => 'apc',
    'prefix' => '',
    'path' => 'sites/default/files/filecache',
    'static' => FALSE,
    'fast_cache' => TRUE,
  ),
);

Things I've done:

Confirmed via phpinfo() that APC is installed and functioning.
Confirmed that AuthCache and Cache Router modules exist in the sites/all/modules directory.
Changed settings.php permissions to 777 to ensure changes are being written to the file.
Tried $conf['cache_inc_via_authcache'] = './sites/all/modules/contrib/cacherouter/cacherouter.inc'; instead, per a previous issue.

I would be very grateful for any help you can offer. Please let me know if there is additional information I can provide.

Thanks!
Joy