Update react-docgen to the latest version 🚀 (#64)

## 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).

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  <summary>FAQ and help</summary>

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package.json

@@ -54,7 +54,7 @@
     "mocha": "^5.2.0",
     "proxyquire": "^2.0.1",
     "react": "^16.5.0",
-    "react-docgen": "^3.0.0-beta12",
+    "react-docgen": "^4.0.0",
     "react-dom": "^16.5.0",
     "react-test-renderer": "^16.5.0",
     "rollup": "^1.0.0",