[build] Set macOS minimum deployment version to 10.13. (#715)
[build] Set macOS minimum deployment version to 10.13. (#715)

macOS 10.13 matches the minimum deployment version of PythonKit.

SwiftPM commands like `swift build` now work without the need to specify
`-Xswiftc -target -Xswiftc x86_64-apple-macosx10.13`.
tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis