Update default URL to api.geocode.earth

[barbeau]

Mapzen Search (and all of Mapzen) was officially announced to be shutting down in Jan 2018:
https://mapzen.com/blog/open-is-closed/

The new default URL should be api.geocode.earth:
https://geocode.earth/

[barbeau]

Locations for default API endpoints in code are:

• https://github.com/CUTR-at-USF/pelias-client-library/blob/master/src/main/java/edu/usf/cutr/pelias/SearchRequest.java#L34
• https://github.com/CUTR-at-USF/pelias-client-library/blob/master/src/main/java/edu/usf/cutr/pelias/AutocompleteRequest.java#L34