[FIX] GAMIFICATION: Fixed the Attribute Error

 Hello,

Fixed the ERROR occured during the cron in **GAMIFICATION**

Stacktrace (most recent call last):
  File "openerp/addons/base/ir/ir_cron.py", line 138, in _callback
    getattr(model, method_name)(cr, uid, *args)
  File "addons/gamification/models/challenge.py", line 279, in _cron_update
    self.write(cr, uid, planned_challenge_ids, {'state': 'done'}, context=context)
  File "addons/gamification/models/challenge.py", line 245, in write
    self.check_challenge_reward(cr, uid, ids, force=True, context=context)
  File "addons/gamification/models/challenge.py", line 709, in check_challenge_reward
    (start_date, end_date) = start_end_date_for_period(challenge.period, challenge.start_date, challenge.end_date)
  File "addons/gamification/models/challenge.py", line 61, in start_end_date_for_period
    return (start_date.strftime(DF), end_date.strftime(DF))

Occuring at **#Sentry id:>  90b5f1350f364fa983546ad75a386033**

Thankz.