keep the shelf when we fall back to RoW feed (bug 1068917)

Changed the ES query to do a ```should``` on the RoW region + the original request region during the fallback.

The region filter used to be a ```must```, but we can use just shoulds: "In a boolean query with no must clauses, one or more should clauses must match a document."
I think you can use `mkt.constants.carriers.get_carrier` for this.