fix reverse geocoding service (#164)

[samwilson]

This looks good to me, and works in my testing. (Not that I'm a maintainer here, just a random user.)

I think in switching to Nominatim there are some other changes that will need to be made, to abide by the Nominatim Usage Policy, probably mostly:

• site-specific user agent;
• rate limiting to 1 request per second; and
• attribution to Nominatim displayed on the OSM keywords admin page.