Fix regex used to parse message ID from message href.

If messages are retrieved via the `GET /v1/queues/demoqueue/messages` API call, the messages' hrefs in the response do not contain a `claim_id` (see http://docs.rackspace.com/queues/api/v1.0/cq-devguide/content/GET_getMessages_v1_queues__queue_name__messages_message-operations.html#GET_getMessages_v1_queues__queue_name__messages_message-operations-Response). So the regex should not assume that `claim_id` will always be present in the message href.