Update header.html

Author: lefticus

_includes/header.html

@@ -77,7 +77,7 @@
 
 <div class="hidden-print">
   <div class="container">
-    <ul class="list-inline">
+    <ul class="list-inline center-block text-center">
       <li>
       <iframe class="github-btn" src="http://ghbtns.com/github-btn.html?user=ChaiScript&repo=ChaiScript&type=watch&count=true" allowtransparency="true" frameborder="0" scrolling="0" width="100px" height="20px"></iframe>
       </li>