add enzyme snapshot serializer to jest

converts all the existing
`expect(component.debug()).toMatchSnapshot()`
to:
`expect(component).toMatchSnapshot()`
which creates more detailed and more diff-able snapshots

test plan:
- specs pass -- this only affects js test code

Change-Id: Ib6dbcc1e3f87e61a251e4635dc68353aca57aeac
Reviewed-on: https://gerrit.instructure.com/167651
Tested-by: Jenkins
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

app/jsx/actAs/__tests__/ActAsModal.test.js

@@ -49,7 +49,7 @@ const props = {
 describe('ActAsModal',  () => {
   it('renders with panda svgs, user avatar, table, and proceed button present', () => {
     const wrapper = shallow(<ActAsModal {...props} />)
-    // expect(wrapper).toMatchSnapshot() // Coming soon.... (need to get enzyme upgraded to 3.x first)
+    expect(wrapper).toMatchSnapshot()
 
     const mask = wrapper.find(ActAsMask)
     const panda = wrapper.find(ActAsPanda)