Edit the rcube_install.php file inside the installer folder and add the 0.6 descriptor to the dropdown array. Like so:
function versions_select($attrib = array()){
$select = new html_select($attrib);
$select->add(array(
'0.1-stable', '0.1.1',
'0.2-alpha', '0.2-beta', '0.2-stable',
'0.3-stable', '0.3.1',
'0.4-beta', '0.4.2',
'0.5-beta', '0.5', '0.5.1',
'0.6-beta', '0.6',
));
return $select;
}
Then re-run the update.