doc: update 'Fork me at Github' ribbon

Replace 'Fork me at Github' ribbon with a new one fitting the website's color
scheme.

doc/index.html

@@ -33,7 +33,7 @@
         <a href="api/" class="button" id="docsbutton">Docs</a>
         <p class="version">__VERSION__</p> 
 
-        <a href="http://github.com/joyent/node"><img class="forkme" src="https://a248.e.akamai.net/camo.github.com/abad93f42020b733148435e2cd92ce15c542d320/687474703a2f2f73332e616d617a6f6e6177732e636f6d2f6769746875622f726962626f6e732f666f726b6d655f72696768745f677265656e5f3030373230302e706e67" alt="Fork me on GitHub"></a>
+        <a href="http://github.com/joyent/node"><img class="forkme" src="images/forkme.png" alt="Fork me on GitHub"></a>
     </div>
     <div id="quotes" class="clearfix">
         <h2>Node.js in the Industry</h2>