Doesnt work for React 16+

https://github.com/btholt/complete-intro-to-react/blob/03296d511993ad5399c09b032d2088583ab13102/package.json#L16

If you are going to use React 16,
Replace `jest-serializer-enzyme` to `enzyme-to-json/serializer` and include `enzyme-to-json` as a dev dependency.

For enzyme to work with React 16, you need to `yarn add enzyme-adapter-react-16 -D`

And then, inside your tests,
```
import { shallow, configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';
beforeAll(() => {
configure({
adapter: new Adapter()
});
});
// your tests here..
```