Allow SwiftPM to use Swift 3 or 4

Note: We can't yet adopt Swift 4 Package format,
since the Swift 3 version of SwiftPM chokes on it.

Author: tbkka

Package.swift

@@ -1,3 +1,4 @@
+// swift-tools-version:3.1
 // Package.swift - description
 //
 // Copyright (c) 2014 - 2016 Apple Inc. and the project authors
@@ -9,6 +10,20 @@
 
 import PackageDescription
 
+#if swift(>=3.1)
+let package = Package(
+  name: "SwiftProtobuf",
+  targets: [
+    Target(name: "PluginLibrary",
+           dependencies: ["SwiftProtobuf"]),
+    Target(name: "protoc-gen-swift",
+           dependencies: ["PluginLibrary", "SwiftProtobuf"]),
+    Target(name: "Conformance",
+           dependencies: ["SwiftProtobuf"]),
+  ],
+  swiftLanguageVersions: [3, 4]
+)
+#else
 let package = Package(
   name: "SwiftProtobuf",
   targets: [
@@ -20,3 +35,4 @@ let package = Package(
            dependencies: ["SwiftProtobuf"]),
   ]
 )
+#endif