[refactored] useMockedRAF to use new sinon stub API

Summary: As directed by a deprecation warning

Reviewers: O2 Material Motion, O3 Material JavaScript platform reviewers, #material_motion, featherless

Reviewed By: O2 Material Motion, #material_motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D3109

packages/testing-utils/src/useMockedRAF.ts

@@ -36,7 +36,7 @@ export default function useMockedRAF(closure) {
 
     before(
       () => {
-        stub(window, 'requestAnimationFrame', mockRAF.raf);
+        stub(window, 'requestAnimationFrame').callsFake(mockRAF.raf);
       }
     );