multi copy of react

When using react-code-splitting@1.2.1 with react@15.6.2, I'm running into Multiple copies of React Error.
I think that's because react should be peerDependency.
```
  "peerDependencies": {
    "react": "^15.3.0 || ^16.0.0",
    "react-dom": "^15.3.0 || ^16.0.0"
  },
```