Update Mac icon to be flatter, to work with Yosemite styling

Yosemite has a much flatter look to it, and all of the default Mac apps have had their icons changed. I updated the Atom icon to fit in with these theme a little better. Any feedback would be appreciated :)


Old: ![atom](https://cloud.githubusercontent.com/assets/464971/3705051/b566c906-141a-11e4-96ea-44397c3e610f.png)
New: ![f2b4523d650ae78d-64x64](https://cloud.githubusercontent.com/assets/464971/3705035/8215410e-141a-11e4-91bb-2c7ffa7c8982.png)