Parse/format example using wrong React/ReactDOM versions

### Are you submitting a **bug report** or a **feature request**?
Bug report

### What is the current behavior?
The `parse`/`format` sandbox example isn't working due to its React/ReactDOM versions. This example is currently using version `16.0.0`. In this one, `React.createContext()` isn't available yet, and that's what's breaking the example.

### What is the expected behavior?
The new context API is available from version `16.3.0`. Updating both React and React DOM versions in the `package.json` file should do it.

### Sandbox Link
https://codesandbox.io/s/10rzowm323

### What's your environment?
```
"dependencies": {
"styled-components": "latest",
"react-final-form": "latest",
"react-dom": "16.0.0",
"react": "16.0.0",
"final-form": "latest"
}
```

### Other information
I would fix this small bug, but I didn't figure it out how to edit the sandbox code. If there's a way, I'd gladly do it.

リンクされた CodeSandbox の parse/format 例を開き、その package.json の依存関係を確認します。React と ReactDOM を 16.0.0 から createContext を提供するバージョンに更新し、その後、報告されたエラーなしで sandbox の例が読み込まれ、動作することを確認します。

索引モデルが issue の本文から書いたものです。