Carthage support

According to their documentation, Carthage needs shared scheme to build dynamic framework.
> Carthage will only build Xcode schemes that are shared from your .xcodeproj. You can see if all of your intended schemes build successfully by running carthage build --no-skip-current, then checking the Carthage.build folder.

Alamofire has scheme that builds dynamic framework, but it is not shared. So I made it shared.
I tested Carthage builds Alamofire.framework correctly on the [forked repository](https://github.com/ishkawa/Alamofire).

NOTE: new tag is required to notify update of this repository to Carthage.