Make API key optional in MapQuest lookup

MapQuest doesn't require a key for the "open" API, but the
geocoder throws an error if you don't provide one anyway.
This edit makes the API key optional.

lib/geocoder/lookups/mapquest.rb

@@ -10,7 +10,7 @@ module Geocoder::Lookup
     end
 
     def required_api_key_parts
-      ["key"]
+      []
     end
 
     def query_url(query)