That patch doesn't fix the problem you are talking about. There isn't a great fix. The value set in the .conf file for authentication type is apparently ignored by the installer program. (NB: the actual program honors it, only the installer ignores it.)
You can hard-code your selection into the program by editing program/include/rcube_imap_generic.php. You can add a line to the authenticate function (line 465):
function authenticate($user, $pass, $type='PLAIN')
{
$type = 'PLAIN';
if ($type == 'CRAM-MD5' || $type == 'DIGEST-MD5') {
It's an ugly fix but it will let you run the installer's tests. I'd remove it after you finish testing, because the actual user login will use the value in the conf file.