Use https to include ghbtns.com iframes.

docs/_includes/social-buttons.html

@@ -1,10 +1,10 @@
 <div class="s2-docs-social">
   <ul class="s2-docs-social-buttons">
     <li>
-      <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=select2&amp;repo=select2&amp;type=watch&amp;count=true" width="100" height="20" title="Star on GitHub"></iframe>
+      <iframe class="github-btn" src="https://ghbtns.com/github-btn.html?user=select2&amp;repo=select2&amp;type=watch&amp;count=true" width="100" height="20" title="Star on GitHub"></iframe>
     </li>
     <li>
-      <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=select2&amp;repo=select2&amp;type=fork&amp;count=true" width="102" height="20" title="Fork on GitHub"></iframe>
+      <iframe class="github-btn" src="https://ghbtns.com/github-btn.html?user=select2&amp;repo=select2&amp;type=fork&amp;count=true" width="102" height="20" title="Fork on GitHub"></iframe>
     </li>
   </ul>
 </div>