How to create group dependency on 7.x-2.6?

I'm trying use two group of migration named prepare and hn. I want make migration of hn group dependant from prepare group. I try this code without success:

/**
 * Implements hook_migrate_api().
 */
function hn_migrate_migrate_api() {
  $api = array(
    'api' => 2,
    'groups' => array(
      'prepare' => array(
        'title' => 'HNonline prepare migration',
      ),
      'hn' => array(
        'title' => 'HNonline',
        'dependencies' => array('prepare'),
      ),
    ),
    'migrations' => array(
      'PrepareAttachment' => array(
        'class_name' => 'PrepareAttachmentMigration',
        'group_name' => 'prepare',
      ),
      'PrepareImage' => array(
        'class_name' => 'PrepareImageMigration',
        'group_name' => 'prepare',
      ),
      'Attachments' => array(
        'class_name' => 'AttachmentsMigration',
        'group_name' => 'hn',
      ),
      'Articles' => array(
        'class_name' => 'ArticlesMigration',
        'group_name' => 'hn',
      ),
    ),
  );
  return $api;
}

How can i do this? Thanks!