Specifying macOs 10.13 as build target.

[vguerra]

The code base uses APIs available from macOS 10.13 hence to build the library one has to:

swift build -Xswiftc -target -Xswiftc x86_64-apple-macosx10.13

Since swift 5 one can use the platforms parameter for Package.init to specify desired target which has to nice advantages:

• one can build the package in macOS w/out passing any flags: swift build.
• The generated Xcode project has the right Deployment Target setting for the Targets.

In order to take advantage of this functionality swift-tools-version needs to bumped. Which is ok since S4TF toolchain is needed to build this pkg.

[vguerra]

Question: I know that this repo is built from the course notebooks, the Package.swift is as well generated automatically or only the files under Sources?

If the entire repo is built automatically, it would be nice to apply this change as well to the conversion code.

[fkautz]

This appears to be necessary to fix build breaks on OSX.

[sgugger]

The Packages.swift file is create manually for now, so this fix can be applied directly. Thanks for flagging!