Fix "double border" of ButtonGroup

Fixes following issue: When resizing the browser window (Chrome) the middle borders jump between a single and double border:

![screen shot 2014-06-17 at 1 24 59 pm](https://cloud.githubusercontent.com/assets/378023/3296399/0d6f98f8-f5da-11e3-867e-83c3c41c20f2.png)

Not quite sure why, maybe a rounding error or so.. Anyways.. this PR should fix it to always have only a single pixel border:

![screen shot 2014-06-17 at 1 41 45 pm](https://cloud.githubusercontent.com/assets/378023/3296409/74c5e1d8-f5da-11e3-9c1d-98f84c5be494.png)