Update Package Description to 4.0 and Spectre version to 0.8.0 (#39)

Author: Keith Smiley

.swift-version

@@ -1 +1 @@
-3.1.1
+4.0

.travis.yml

@@ -4,7 +4,7 @@ os:
 language: generic
 sudo: required
 dist: trusty
-osx_image: xcode8.3
+osx_image: xcode9
 install:
 - eval "$(curl -sL https://gist.githubusercontent.com/kylef/5c0475ff02b7c7671d2a/raw/9f442512a46d7a2af7b850d65a7e9bd31edfb09b/swiftenv-install.sh)"
 script:

Package.resolved

@@ -1,9 +1,17 @@
+// swift-tools-version:4.0
 import PackageDescription
 
 let package = Package(
   name: "PathKit",
+  products: [
+    .library(name: "PathKit", targets: ["PathKit"]),
+  ],
   dependencies: [
-    // https://github.com/apple/swift-package-manager/pull/597
-    .Package(url: "https://github.com/kylef/Spectre.git", majorVersion: 0, minor: 7),
-  ]
+    .package(url:"https://github.com/kylef/Spectre.git", .upToNextMinor(from:"0.8.0"))
+  ],
+  targets: [
+    .target(name: "PathKit", dependencies: [], path: "Sources"),
+    .testTarget(name: "PathKitTests", dependencies: ["PathKit", "Spectre"], path:"Tests/PathKitTests")
+  ],
+  swiftLanguageVersions: [3]
 )