[IMP] user_story: update module api from v7 to v8

Created by: zaoral

module: user_story version: 2.6

This module is using api v7. But this reason when runing travis with LINK_CHECK that review the "odoo lints" return a red.

For example, api v7 use the _defaults attribute in the definition of the class. But this is mark as a error by odoo lint.

Best Regards.