Add :place_id attribute to Spot.

Posted on Google Places API as of June 24, 2014:

Note: The id and reference fields are deprecated as of June 24, 2014. They are replaced by the new place ID, a unique identifier that can be used to compare places and to retrieve information about a place. The Places API currently returns a place_id in all responses, and accepts a place_id in the Place Details and Place Delete requests. Soon after June 24, 2015, the API will stop returning the id and reference fields in responses. Some time later, the API will no longer accept the reference in requests. We recommend that you update your code to use the new place_id instead of id and reference as soon as possible.