append build revision to stylesheet path to break cache

_layouts/default.html

@@ -5,7 +5,7 @@
     <meta http-equiv="X-UA-Compatible" content="chrome=1">
     <title>{{ site.title | default: site.github.repository_name }} by {{ site.github.owner_name }}</title>
 
-    <link rel="stylesheet" href="{{ '/assets/css/style.css' | relative_url }}">
+    <link rel="stylesheet" href="{{ '/assets/css/style.css?v=' | append: site.github.build_revision | relative_url }}">
     <meta name="viewport" content="width=device-width">
     <!--[if lt IE 9]>
     <script src="//html5shiv.googlecode.com/svn/trunk/html5.js"></script>