It is no longer considered good practice to use protocol-relative URLs (http://www.paulirish.com/2010/the-protocol-relative-url/). So the CDN request should be made via HTTPs.
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Paste the following code into the <code><head></code> section of your site's HTML.
{% highlight html %}
-<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/font-awesome/{{ site.fontawesome.version }}/css/font-awesome.min.css">
+<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/{{ site.fontawesome.version }}/css/font-awesome.min.css">
{% endhighlight %}
<p class="alert alert-success"><i class="fa fa-info-circle"></i> Immediately after release, it takes a bit of time for BootstrapCDN to catch up and get the newest version live on their CDN.</p>
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