Fix embarrassing typo caught by IE8.

packages/deps/deps.js

@@ -58,7 +58,7 @@ if (typeof Sky === "undefined") Sky = {};
         _.each(ctx._callbacks, function (f) {
           f(); // XXX wrap in try?
         });
-        delete this._callbacks; // maybe help the GC
+        delete ctx._callbacks; // maybe help the GC
       });
     },
 
@@ -66,4 +66,4 @@ if (typeof Sky === "undefined") Sky = {};
       Context: Context
     }
   });
-})();
+})();