Update Geonames license

They use CC-BY-4.0 now

public/attribution.md

@@ -3,5 +3,5 @@
 * Data from
    * [OpenStreetMap](http://www.openstreetmap.org/copyright) © OpenStreetMap contributors under [ODbL](http://opendatacommons.org/licenses/odbl/)
    * [OpenAddresses](http://openaddresses.io) under a [Creative Commons Zero](https://github.com/openaddresses/openaddresses/blob/master/sources/LICENSE) public domain designation
-   * [GeoNames](http://www.geonames.org/) under [CC-BY-3.0](https://creativecommons.org/licenses/by/2.0/)
+   * [GeoNames](http://www.geonames.org/) under [CC-BY-4.0](https://creativecommons.org/licenses/by/4.0/)
    * [WhosOnFirst](https://www.whosonfirst.org/) under [various licenses](https://github.com/whosonfirst/whosonfirst-data/blob/master/LICENSE.md)