Open Amazon redeem link in system browser. Fix buy gc on mobile

public/views/amazon.html

@@ -10,7 +10,7 @@
     </section>
 
     <section class="right-small">
-      <a class="p10" ng-click="$root.go('buyAmazon')">
+      <a class="p10" ui-sref="buyAmazon">
         <i class="fi-shopping-cart size-24"></i>
       </a>
     </section>
@@ -66,7 +66,7 @@
       <div class="row">
         <div class="columns">
           <button class="m20t button black round expand" 
-                  ng-click="$root.go('buyAmazon')">
+                  ui-sref="buyAmazon">
             Buy now
           </button>
         </div>