Fix 4dcdfcc xcworkspace.

Continuing #11 & #12.

Drag-and-dropping Carthage.checkout folder to xcworkspace as "Create folder references for any added folders" will mix up OSX/iOS xcodebuild when `carthage build` from depending project e.g. [ReactKit](https://github.com/ReactKit/ReactKit).

```
Build settings from command line:
    SDKROOT = iphoneos8.1

=== BUILD TARGET SwiftState-OSX OF PROJECT SwiftState WITH CONFIGURATION Release ===

Check dependencies
CodeSign error: code signing is required for product type 'Framework' in SDK 'iOS 8.1'
```