The active `Listbox` option doesn't scroll into view when using keyboard arrows

[barryofguilder]

It seems that when you render a Listbox with enough items where the menu scrolls, using the keyboard arrows to navigate through the options list won't scroll the active option into view.

To reproduce, I copied the contents of the listbox-with-transition.hbs example and added enough options so that the menu is scrollable. When I navigate the menu using the arrow keys, the active option isn't scrolled to. I have to manually use my mouse to scroll it into view.