Replace backslashes with forward slashes in URL

### hello there

Hello @maddox! i come in :v: ("peace", there's no peace emoji?). i've been lurking around looking at atom-shell-based repos for my programmer-play-time and seeing yours, it brought me back to fun times with MAME during college. So i've been playing around to try to get it going on my box.

:warning: i don't know what i'm doing when it comes to node/js/coffeescript/Spine/Atom etc... so please feel free to let me know if i'm completely missing things :smile:

:warning: i'm on a windows box too, so yeah, there's lots of wonky stuff going on, i may add more PRs to help other windows people like me.

### skip to here for actual PR content

This fixes the file URL on a windows machine. i was getting:

![noimage](https://cloud.githubusercontent.com/assets/238079/3935178/cb1e6f0a-2492-11e4-8784-56796abd34a1.PNG)

and seeing `GET file:///C:/Usersjames_000ocumentssrckartppimages%0Bg-texture.png` isn't fun.

This patch brings that nice background back to me.