Update react-docgen to the latest version 🚀 (#165)

## The devDependency [react-docgen](https://github.com/reactjs/react-docgen) was updated from `3.0.0` to `4.0.0`.
This version is **not covered** by your **current version range**.

If you don’t accept this pull request, your project will work just like it did before. However, you might be missing out on a bunch of new features, fixes and/or performance improvements from the dependency update.

---

[Find out more about this release](https://github.com/reactjs/react-docgen).

<details>
  <summary>FAQ and help</summary>

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package.json

@@ -73,7 +73,7 @@
     "mocha": "^5.2.0",
     "npm-run-all": "^4.1.3",
     "react": "^16.4.2",
-    "react-docgen": "^3.0.0",
+    "react-docgen": "^4.0.0",
     "react-dom": "^16.4.2",
     "react-test-renderer": "^16.4.2",
     "rollup": "^1.0.0",