Fix tooltip arrow border clipping.

I missed the border clip by 1px in https://github.com/orderedlist/unity-ui/pull/21 so it bleeds into the tooltip area. It's easier to spot it now that I'm looking at my retina display.

Before:
![before](https://cloud.githubusercontent.com/assets/122102/3692218/89739808-135a-11e4-9631-392b2ead37ea.png)

After:
![after](https://cloud.githubusercontent.com/assets/122102/3692217/8969fe88-135a-11e4-8160-9bf8b2e4c482.png)