vows.mongo_storage_vows: Set a server in GetNothingAfterExpiration

Avoid:

    Traceback (most recent call last):
      File "/…/pyvows/runner/gevent.py", line 104, in _run_setup_and_topic
        topic = topic_func(*topic_list)
      File "/…/vows/mongo_storage_vows.py", line 210, in topic
        storage.put(IMAGE_URL % 10, IMAGE_BYTES)
      File "/…/thumbor/storages/mongo_storage.py", line 39, in put
        if not self.context.server.security_key:
    AttributeError: 'NoneType' object has no attribute 'security_key'