Update Swift Package Manager manifest and Dockerfile to Swift 4.0.

Add note regarding version compatibility to the README.

Dockerfile

@@ -1,5 +1,5 @@
 # Build and test Open Location Code for Swift in Linux.
-FROM swift:3.1
+FROM swift:4.0
 RUN mkdir /OpenLocationCode
 WORKDIR /OpenLocationCode
 ADD . /OpenLocationCode

Package.swift

@@ -1,3 +1,5 @@
+// swift-tools-version:4.0
+
 //===-- OpenLocationCodeTests.swift - Tests for OpenLocationCode.swift ----===//
 //
 //  Copyright 2017 Google Inc.
@@ -20,13 +22,27 @@
 //
 //===----------------------------------------------------------------------===//
 
-// swift-tools-version:3.1
-
 import PackageDescription
 
-let package = Package(
+let packages = Package(
     name: "OpenLocationCode",
-    exclude: ["Framework",
-              "Tests/OpenLocationCodeObjCTests",
-              "Source/OpenLocationCodeObjCBridge.swift"]
+    products: [
+        .library(
+            name: "OpenLocationCode",
+            targets: ["OpenLocationCode"]),
+    ],
+    targets: [
+        .target(
+            name: "OpenLocationCode",
+            path: ".",
+            exclude: ["Source/OpenLocationCodeObjCBridge.swift", "Framework", "Tests", "test_data"],
+            sources: ["Source"]
+        ),
+        .testTarget(
+            name: "OpenLocationCodeTests",
+            dependencies: ["OpenLocationCode"],
+            path: "Tests",
+            exclude: ["OpenLocationCodeObjCTests"]
+        )
+    ]
 )

README.md

@@ -40,6 +40,14 @@ This library is provided as a Swift and Objective-C Cocoa Framework
 for iOS, macOS, tvOS and watchOS, and as a pure Swift module
 for macOS and Linux.
 
+## Swift Versions
+
+* Versions 1.x of Open Location Code for Swift are designed for Swift 3.2.
+* Versions 2.x of Open Location Code for Swift are designed for Swift 4.0.
+
+Choose the latest 1.x release for greater compatability, or the latest 2.x
+release if you're fully in the Swift 4 world!
+
 ## Building
 
 ### Cocoa Framework