speed up topbar animation for responsive

docs/assets/css/bootstrap-responsive.css

@@ -374,6 +374,7 @@
   .nav-collapse {
     overflow: hidden;
     height: 0;
+    -webkit-transform: translate3d(0, 0, 0);
   }
 }
 @media (min-width: 940px) {

less/responsive.less

@@ -278,6 +278,7 @@
   .nav-collapse {
     overflow: hidden;
     height: 0;
+    -webkit-transform: translate3d(0, 0, 0); // activate the GPU
   }
 }