Use Swift 4 Package Description API

Package.swift

@@ -1,5 +1,22 @@
+// swift-tools-version:4.0
+
+/**
+ *  Require
+ *  Copyright (c) John Sundell 2017
+ *  Licensed under the MIT license. See LICENSE file.
+ */
+
 import PackageDescription
 
 let package = Package(
-    name: "Require"
+    name: "Require",
+    products: [
+        .library(name: "Require", targets: ["Require"])
+    ],
+    targets: [
+        .target(
+            name: "Require",
+            path: "Sources"
+        )
+    ]
 )