Alert type wasn't changing

So this is the before: http://embed.plnkr.co/LKrbpPAY0rC07TgfsohS/preview

As you click each button you notice that the alert doesn't change colour, however if you inspect the HTML you can see the `ng-class` attribute changing. Sad times.

And this is the after: http://embed.plnkr.co/wB5SCSWdv3iC5lyY2daj/preview

Now as you click the buttons the colour changes, good times!