Fix warnings in canvas.

components/canvas/lib.rs

@@ -2,8 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![feature(core)]
+
 #![allow(missing_copy_implementations)]
-#![allow(unstable)]
 
 extern crate azure;
 extern crate geom;