Use jBuilder for JSON API, upgrade gems to newest versions

Overiding as_json can lead to strange and unexpected results
(like exposing fields that should be a secret). Since
the real task is templating, use a json templating lib
Depreciate Retailer#distance_from_origin to enable refactor for RetailerController#index

Instead of risking breaking current functionality, make a clean
break and use new methods to generate distances.
Render retailers as text, refactor RetailerControllerTest