Update script versions in Readme.md

[nikolasleblanc]

Struggled getting firefox 57.0.1 (and developer edition) to handle a React component with prop.children (react-dom complained about element), so tried bumping the versions of the webcomponent polyfills listed in the readme and it all worked.

<script src="https://cdnjs.cloudflare.com/ajax/libs/webcomponentsjs/1.0.20/custom-elements-es5-adapter.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/webcomponentsjs/1.0.20/webcomponents-sd-ce.js"></script>