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."
![filthy animal](https://cloud.githubusercontent.com/assets/23885/4326727/4622c21a-3f72-11e4-8855-b04c3c907c37.gif)