Upgrade React 16

[rwieruch]

The 4th edition of the book uses React 16. If you started to read the book in a previous edition, but grabbed the recent 4th edition, you need to upgrade your dependencies in your package.json file. The easiest way to do so is executing the following commands on the command line in your project:

npm i -g npm-check-updates
npm-check-updates -u
npm install

It is a global npm module that can be used to update your dependencies, such as react or react-dom, in your package.json file. I should’ve mentioned it in the first place, for people grabbing the new edition who have an old React version installed.

See also #83

[rwieruch]

Closing this because React 16 was a long time ago and I don't expect people to run into this anymore.