Use input event not keyup

The `oninput` event is a more performant way to detect changes to a
textbox’s contents (compared to `onkeydown`, `onpaste`, etc).

It’s not supported in older browsers, but since this is a likely to be
used by platform admin users only that’s OK.

app/assets/javascripts/colourPreview.js

@@ -15,7 +15,7 @@
       );
 
       this.$input
-        .on('change keyup', this.update)
+        .on('input', this.update)
         .trigger('change');
 
     };