Fix deprecation in selector matching test

The 'find' function was changed to 'get'. Changed to reflect this
warning given when running the tests.
auto merge of #4097 : orteipid/servo/fix_deprecation, r=SimonSapin

    selector_matching.rs:1263:44: 1263:63 warning: use of deprecated item: Renamed to `get`, #[warn(deprecated)] on by default
    selector_matching.rs:1263         assert_eq!(1, selector_map.id_hash.find(&atom!("top")).unwrap()[0].declarations.source_order);
                                                                         ^~~~~~~~~~~~~~~~~~~
    selector_matching.rs:1265:47: 1265:79 warning: use of deprecated item: Renamed to `get`, #[warn(deprecated)] on by default
    selector_matching.rs:1265         assert_eq!(0, selector_map.class_hash.find(&Atom::from_slice("intro")).unwrap()[0].declarations.source_order);
                                                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    selector_matching.rs:1266:41: 1266:71 warning: use of deprecated item: Renamed to `get`, #[warn(deprecated)] on by default
    selector_matching.rs:1266         assert!(selector_map.class_hash.find(&Atom::from_slice("foo")).is_none());
                                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The 'find' function was changed to 'get'. Updated to remove this warning given when running the tests.