Update poll bookmark after getting latest shipments

The /get_shipments hook returns the same results every time. It doesn't bump the last poll timestamp up because `Bookmark = 2`.

Setting `Bookmark = 3` fixes the issue, although there are some edge cases:

* The ShipWire API slows up and the wombat connection times out, but the ShipWire API request completes successfully. Wombat doesn't get the shipment notifications but the ShipWire bookmark is bumped up
* There is a bug later in the code which causes an exception to be raised.