Specify minimum deployment target for Swift Package Manager, fixes Issue #328

Package.swift

@@ -28,6 +28,9 @@ import PackageDescription
 
 let package = Package(
     name: "CoreStore",
+    platforms: [
+           .macOS(.v10_12), .iOS(.v10), .tvOS(.v10), .watchOS(.v3)
+    ],
     products: [
         .library(name: "CoreStore", type: .static, targets: ["CoreStore"])
     ],