Include only locations having a zip code in calculating map bounds.
Include only locations having a zip code in calculating map bounds.

Solves #229. Including locations without zip codes in calculating bounds
causes the map to zoom out to its default view. We need zip codes to
calculate bounds because this happens before geocoding results have
arrived from the Maps API.

votinginfoproject/vip-embeddable-tool,votinginfoproject/vip-embeddable-tool,votinginfoproject/vip-embeddable-tool