Remove scrollbars when not needed

I forgot that, on Windows devices, `overflow: scroll;` will always force
scrollbars even if they’re not needed. `auto` solves that.

Author: csswizardry

demo/index.html

@@ -171,7 +171,7 @@
       color: #829396;
       white-space: pre;
       border-radius: 4px;
-      overflow: scroll;
+      overflow: auto;
       text-transform: none;
       padding-right: 2.5em;
     }