Deprecate protocol-relative URLs

Always use the HTTPS URL when it’s available.

See also:
* https://github.com/konklone/cdns-to-https#conclusion-cdns-should-redirect-to-https
* http://www.paulirish.com/2010/the-protocol-relative-url/

Close h5bp#1694

doc/html.md

@@ -145,8 +145,8 @@ using some polyfill CDN service, like [cdn.polyfill.io](https://cdn.polyfill.io/
 just put it before the other scripts in the bottom of the page:
 
 ```html
-    <script src="//cdn.polyfill.io/v1/polyfill.min.js"></script>
-    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
+    <script src="https://cdn.polyfill.io/v1/polyfill.min.js"></script>
+    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
     <script>window.jQuery || document.write('<script src="js/vendor/jquery-1.11.2.min.js"><\/script>')</script>
     <script src="js/plugins.js"></script>
     <script src="js/main.js"></script>

index.html

@@ -22,7 +22,7 @@
         <!-- Add your site or application content here -->
         <p>Hello world! This is HTML5 Boilerplate.</p>
 
-        <script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
+        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
         <script>window.jQuery || document.write('<script src="js/vendor/jquery-1.11.2.min.js"><\/script>')</script>
         <script src="js/plugins.js"></script>
         <script src="js/main.js"></script>