remove unwanted icon artifacts

shuttle icon has some artifacts that are really visible in retina displays

![screen shot 2015-10-30 at 2 50 41 pm](https://cloud.githubusercontent.com/assets/4992951/10859002/733c6244-7f16-11e5-8286-d25c0a04124f.png)

zoomed
<img width="191" alt="screen shot 2015-10-30 at 2 51 20 pm" src="https://cloud.githubusercontent.com/assets/4992951/10859005/78c21a10-7f16-11e5-8e69-72553406adc0.png">
