Change dropbutton arrow to fit with other arrows in app

Currently the dropbutton arrow looks like this:

![screen shot 2014-08-27 at 10 09 50 am](https://cloud.githubusercontent.com/assets/2559233/4060909/d93f3334-2df3-11e4-901f-f677d6b474ad.png)

The arrow doesn't fit with the branch dropdown or a lot of other aspects of Atlas, so this changes the drop arrow to look like

![screen shot 2014-08-27 at 10 09 17 am](https://cloud.githubusercontent.com/assets/2559233/4060918/ec919fda-2df3-11e4-92ca-39071f85a8d8.png)