Upgrades Draft to React 16.8 ⚛️ (facebookarchive#1996)

Summary:
**Summary**

Upgrades [react](https://npm.im/react) and [react-dom](https://npm.im/react-dom) dependencies to the freshly released 16.8 version supporting Hooks.

**Test Plan**

`yarn`
manual smoke test of editor in examples
Pull Request resolved: facebookarchive#1996

Differential Revision: D13982016

fbshipit-source-id: b2f87a1d85336f60d7a03318e5e0155bb3c44fc8

Author: claudiopro

meta/bundle-size-stats/Draft.js.json

@@ -73,8 +73,8 @@
     "gulp-util": "^3.0.6",
     "jest": "^23.5.0",
     "prettier": "1.13.6",
-    "react": "^16.0.0",
-    "react-dom": "^16.0.0",
+    "react": "^16.8.0",
+    "react-dom": "^16.8.0",
     "react-test-renderer": "^16.0.0",
     "stats-webpack-plugin": "^0.6.2",
     "through2": "^2.0.1",