update sponsor to gridgum

yeti/index.html

@@ -101,7 +101,7 @@
           </div>
           <div class="col-lg-4 col-md-5 col-sm-6">
             <div class="sponsor">
-              <a href="http://www.shopify.com/?ref=bootswatch" target="_blank" onclick="_gaq.push(['_trackEvent', 'banner', 'click', 'shopify']);"><img src="../assets/img/shopify.png" alt="Shopify" onload="_gaq.push(['_trackEvent', 'banner', 'impression', 'shopify']);"></a>
+              <a href="http://gridgum.com/themes/category/bootstrap-themes/?utm_source=Bootswatch&utm_medium=250ad&utm_campaign=Bootswatch%20Campaign" target="_blank" onclick="_gaq.push(['_trackEvent', 'banner', 'click', 'gridgum']);"><img src="../assets/img/gridgum.png" alt="Gridgum" onload="_gaq.push(['_trackEvent', 'banner', 'impression', 'gridgum']);"></a>
             </div>
           </div>
         </div>