Update React boilerplate. (egoist#38)

* Update React boilerplate.

Updates the react boilerplate to use the latest released version from npm.

* Use React 16 instead of latest

Author: thysultan

src/boilerplates/react/codepan.html

@@ -1,4 +1,4 @@
-<script src="https://unpkg.com/react@15/dist/react.js"></script>
-<script src="https://unpkg.com/react-dom@15/dist/react-dom.js"></script>
+<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
+<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
 
 <div id="app"></div>