Use supported name for example 3 and upgrade to Swift 5.0

[FrankenApps]

This would Fix #4.

Also I changed the name of the package from 3-packaging-a-mac-app to packaging-a-mac-app, because cargo does not allow package names beginning with a digit.

I am not sure if this project is still maintained / pull requests are welcome, but I thought it might help someone who is looking into getting this working nowadays...