Replace core-js with polyfill.io

core-js's v3 release removed the `shim.js` file, breaking polyfilling because a version was not specified in the URL.

docs/_includes/scripts.html

@@ -1,7 +1,7 @@
 {% if page.requirejs %}
 <script data-main="{{ site.baseurl }}/js/require-main.js" src="https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.3/require.min.js"></script>
 {% else %}
-<script src="https://unpkg.com/core-js/client/shim.min.js"></script>
+<script crossorigin="anonymous" src="https://polyfill.io/v3/polyfill.min.js?features=default%2Ces5%2Ces6%2Ces7"></script>
 <!-- Syntax highlighting via Prism -->
 <script src="{{ site.baseurl }}/js/prism.js"></script>
 <!-- Shuffle! -->

docs/js/require-main.js

@@ -2,7 +2,7 @@ requirejs.config({
   baseUrl: window.site_url + '/js',
   paths: {
     shufflejs: '../dist/shuffle',
-    polyfill: 'https://unpkg.com/core-js/client/shim.min',
+    polyfill: 'https://polyfill.io/v3/polyfill.min.js?features=default%2Ces5%2Ces6%2Ces7',
   },
 
   // Load the polyfill before Shuffle.