updating call to index_name

This is now stored in the tanker_config object, rather than directly on
the class.

lib/tanker/utilities.rb

@@ -6,7 +6,7 @@ def get_model_classes
       end
 
       def get_available_indexes
-        get_model_classes.map{|model| model.index_name}.uniq.compact
+        get_model_classes.map{|model| model.tanker_config.index_name}.uniq.compact
       end
 
       def clear_all_indexes