Compile with Swift 4.2 if possible

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Package.swift

@@ -1,22 +1,22 @@
-// swift-tools-version:4.0
+// swift-tools-version:4.1
 import PackageDescription
 
 let package = Package(
   name: "Stencil",
   products: [
-    .library(name: "Stencil", targets: ["Stencil"]),
+    .library(name: "Stencil", targets: ["Stencil"])
   ],
   dependencies: [
     .package(url: "https://github.com/kylef/PathKit.git", from: "0.9.0"),
-    .package(url: "https://github.com/kylef/Spectre.git", from: "0.9.0"),
+    .package(url: "https://github.com/kylef/Spectre.git", from: "0.9.0")
   ],
   targets: [
     .target(name: "Stencil", dependencies: [
-      "PathKit",
+      "PathKit"
     ], path: "Sources"),
     .testTarget(name: "StencilTests", dependencies: [
       "Stencil",
-      "Spectre",
+      "Spectre"
     ])
   ]
 )

Package@swift-4.2.swift

@@ -0,0 +1,23 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+  name: "Stencil",
+  products: [
+    .library(name: "Stencil", targets: ["Stencil"])
+  ],
+  dependencies: [
+    .package(url: "https://github.com/kylef/PathKit.git", from: "0.9.0"),
+    .package(url: "https://github.com/kylef/Spectre.git", from: "0.9.0")
+  ],
+  targets: [
+    .target(name: "Stencil", dependencies: [
+      "PathKit"
+    ], path: "Sources"),
+    .testTarget(name: "StencilTests", dependencies: [
+      "Stencil",
+      "Spectre"
+    ])
+  ],
+  swiftLanguageVersions: [.v4, .v4_2]
+)

Stencil.podspec.json

@@ -24,7 +24,7 @@
     "tvos": "9.0"
   },
   "cocoapods_version": "1.4.0",
-  "swift_version": "4.1",
+  "swift_version": "4.2",
   "requires_arc": true,
   "dependencies": {
     "PathKit": [