Update Package.swift to Support Swift Package Manager

[MarcusRobbins]

Re-creating this ticket because the soln was never merged: #275

"_In order to use Swift Package Manager to add Dollar as a dependency in Xcode 11, the Package.swift file must specify at least one target.

This PR updates Package.swift to add Dollar as a target._"

// swift-tools-version:5.1
import PackageDescription

let package = Package(
    name: "Dollar",
    platforms: [
        .iOS(.v9)
    ],
    products: [
        .library(name: "Dollar", targets: ["Dollar"])
    ],
    targets: [
        .target(name: "Dollar", path: "Sources")
    ]
)

Works well for me.

[ankurp]

Taking a look. My last comment was to add additional platforms which I can do.