Error first time tests are run locally

[cesarob]

The first time the tests are run this error happends:

======================================================================
FAIL: test_last_activity (test.test_board.TrelloBoardTestCase)
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Traceback (most recent call last):
  File "/py-trello/test/test_board.py", line 195, in test_last_activity
    self.assertIsInstance(self._board.date_last_activity, datetime)
AssertionError: None is not an instance of <class 'datetime.datetime'>

Second time it works.