Use update_index instead of save when updating

We need to trigger an update of the parent object's index when updating
Supplemental Files. This can be done more cleanly by using
`update_index` method instead of saving the parent object.

app/controllers/supplemental_files_controller.rb

@@ -110,7 +110,8 @@ def update
     @supplemental_file.attach_file(attachment) if attachment
 
     raise Avalon::SaveError, @supplemental_file.errors.full_messages unless @supplemental_file.save
-    raise Avalon::SaveError, @object.errors[:supplemental_files_json] unless @object.save
+    # Updates parent object's solr document
+    @object.update_index
 
     flash[:success] = "Supplemental file successfully updated."
     respond_to do |format|