Amendments to Package manifests

[daveverwer]

Hi! I'd love to add this package to the Swift Package Index, but currently the Package.swift file does not parse using swift package dump-package, which is what we use to gather metadata from the manifest.

I noticed you have a Swift 4 manifest that works, and wondered if it might be appropriate to move that across to be the primary manifest.

Thanks for considering the PR!

Pull Request Checklist

• I've added the default copyright header to every new file.
• Every new file has been correctly indented, no tabs, 4 spaces (you can use swiftlint).
• You've read the contribution guidelines.

I have not added the copyright header as it wasn't there previously. I can add it if you would like, though.

[daveverwer]

I picked 5.6 here as it's the lowest version we support on the package index. Of course, please feel free to reduce this to whatever you'd like to still support.

The only requirement we have is that swift package dump-package runs with the latest stable toolchain.

[uraimo]

Hi Dave, thanks! That was leftover from the Swift 3 days, I will merge this and lower it to 5.1 since there are still some users with that swift build.

[daveverwer]

Thanks for the super speedy review and merge! I'll add it to the index now.