Updated Package.swift to 4.2 definition version

Package.swift

@@ -1,5 +1,15 @@
+// swift-tools-version:4.2
+
 import PackageDescription
 
 let package = Package(
     name: "KeychainSwift"
+    products: [
+        .library(name: "KeychainSwift", targets: ["KeychainSwift"]),
+    ],
+    dependencies: [
+    ],
+    targets: [
+        .target(name: "KeychainSwift", dependencies: [], path: "Sources"),
+    ]
 )