[FIX] membership: recompute state

Commit https://github.com/odoo/odoo/commit/2b587d34ff30147c03baf16e998effe46ce5d9cf#diff-2d656902930900b836412f086c924643R164
incorrectly removed the content of the cron.

Indeed, `membership_state` is a computed field which depends on time
(which is usually a bad idea). Therefore, we must force its
recomputation.

opw-1878878

addons/membership/models/partner.py

@@ -161,8 +161,10 @@ class Partner(models.Model):
 
     @api.model
     def _cron_update_membership(self):
-        # used to recompute 'membership_state'; should no longer be necessary
-        pass
+        partners = self.search([('membership_state', 'in', ['invoiced', 'paid'])])
+        # mark the field to be recomputed, and recompute it
+        partners._recompute_todo(self._fields['membership_state'])
+        self.recompute()
 
     @api.multi
     def create_membership_invoice(self, product_id=None, datas=None):