Fix broken Placeholder.js links

It appears as though GitHub has stopped serving the Placeholder.js
GitHub Pages site.  This broke placeholder support on older versions
of IE in the documentation and broke the link we had to the
polyfill.  Since the repository is still online, and the
documentation should work without an internet connection, the
JavaScript files have been pulled into the documentation folder.

docs/examples.html

@@ -4,7 +4,7 @@ title: Examples - Select2
 slug: examples
 ---
 
-<script type="text/javascript" src="https://jamesallardice.github.io/Placeholders.js/assets/js/placeholders.jquery.min.js"></script>
+<script type="text/javascript" src="vendor/js/placeholders.jquery.min.js"></script>
 <script type="text/javascript" src="dist/js/i18n/es.js"></script>
 
 <div class="container">
@@ -93,7 +93,7 @@ $(".js-example-basic-multiple").select2();
       <p>
         Select2 uses the <code>placeholder</code> attribute on multiple select
         boxes, which requires IE 10+. You can support it in older versions with
-        <a href="https://jamesallardice.github.io/Placeholders.js/">the Placeholders.js polyfill</a>.
+        <a href="https://github.com/jamesallardice/Placeholders.js">the Placeholders.js polyfill</a>.
       </p>
 
       <p>