Update the header with new version number

Author: mrmrs

src/templates/header.html

@@ -4,7 +4,7 @@
       <a href="/" class="dib f5 f4-ns fw6 mt0 mb1 link black-70" title="Home">
         Tachyons
         <div class="dib">
-          <small class="nowrap f6 mt2 mt3-ns pr2 black-70 fw2">v4.7.0</small>
+          <small class="nowrap f6 mt2 mt3-ns pr2 black-70 fw2">v4.8.0</small>
         </div>
       </a>
     </div>