update swift package and bump the versions to ios11, tvOS11,macOS1013,watchOS4

Author: zihejia

Package.swift

@@ -5,10 +5,10 @@ import PackageDescription
 let package = Package(
     name: "Mixpanel",
     platforms: [
-      .iOS(.v9),
-      .tvOS(.v9),
-      .macOS(.v10_10),
-      .watchOS(.v3)
+      .iOS(.v11),
+      .tvOS(.v11),
+      .macOS(.v10_13),
+      .watchOS(.v4)
     ],
     products: [
         .library(name: "Mixpanel", targets: ["Mixpanel"])