Remove 'implements' keyword in favour of :, part of #2301.

src/test/compile-fail/class-method-missing.rs

@@ -3,7 +3,7 @@ iface animal {
   fn eat();
 }
 
-class cat implements animal {
+class cat : animal {
   let meows: uint;
   new(in_x : uint) { self.meows = in_x; }
 }