Change reference to place_id for single Spot fetch

"The id and reference fields are deprecated as of June 24, 2014"
According to the [documentation](https://developers.google.com/places/documentation/search) the id and reference keys are going to be removed. Spot.find method should use the place_id to fetch a Spot instead of the reference.

Since many developers may still count on reference and id (if that's what they store in their database for example), it is not a good option to completely remove the support for reference. Whatever the solution will be, the place_id will be the default so changes will be needed to make it work with reference key.