Truncate extents of loaded items

In the case of a feature with ridiculous coordinate precision, geojson-extent gives us a ridiculously precise bbox. The bounds are passed into dynamodb as `west`, `south`, `east` and `north` attributes, but if given that much precision dynamo bails with a `ValidationError: Number underflow` error. 

I think it should be fine truncating these values to 6 decimal places: they are only used when computing the bbox for a dataset and so they don't need to be terribly precise.