[#111,elasticsearch,geofilter][xs]: work out that what ES docs describe as distance_unit field is actually called unit.

src/backend/elasticsearch.js

@@ -124,6 +124,7 @@ this.recline.Backend.ElasticSearch = this.recline.Backend.ElasticSearch || {};
       } else if (filter.type === 'geo_distance') {
         out.geo_distance[filter.field] = filter.point;
         out.geo_distance.distance = filter.distance;
+        out.geo_distance.unit = filter.unit;
       }
       return out;
     },

src/model.js

@@ -424,7 +424,7 @@ my.Query = Backbone.Model.extend({
     },
     geo_distance: {
       distance: 10,
-      distance_unit: 'km',
+      unit: 'km',
       point: {
         lon: 0,
         lat: 0