Updated swift tools version and platform.

Updated swift tools version to 5.1 and platform to iOS 11.

Author: TimJonsson

Package.swift

@@ -1,8 +1,11 @@
-// swift-tools-version:4.2
+// swift-tools-version:5.1
 import PackageDescription
 
 let package = Package(
     name: "Embassy",
+    platforms: [
+        .iOS(.v11),
+    ],
     products: [.library(name: "Embassy", targets: ["Embassy"])],
     targets: [.target(name: "Embassy", path: "./Sources")]
 )