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'
Merge pull request #365 from wking/mongo-vow-get-nothing-server

vows.mongo_storage_vows: Set a server in GetNothingAfterExpiration