order derivable traits lists

Ignoring :
 - **generated**.rs
 - python/tidy/servo_tidy_tests/rust_tidy.rs

components/script/dom/htmlheadingelement.rs

@@ -10,7 +10,7 @@ use dom::node::Node;
 use dom_struct::dom_struct;
 use html5ever::{LocalName, Prefix};
 
-#[derive(JSTraceable, HeapSizeOf)]
+#[derive(HeapSizeOf, JSTraceable)]
 pub enum HeadingLevel {
     Heading1,
     Heading2,