Update for Swift 4.2 (lyft#148)

* Update for Swift 4.2

There aren't any required syntax changes here, but I have updated the
Package.swift for breaking SwiftPM changes. Without this change you
cannot compile Mapper in Swift 4.2 mode if you depend on it with SwiftPM

* Build for iOS and tvOS 12

.travis.yml

@@ -16,7 +16,7 @@ matrix:
       env: ACTION=swiftpm-macOS
 
 language: objective-c
-osx_image: xcode9.3
+osx_image: xcode10
 
 script:
   - make install-$ACTION && make test-$ACTION

Makefile

@@ -24,7 +24,7 @@ test-iOS:
 		-project Mapper.xcodeproj \
 		-scheme Mapper \
 		-configuration Release \
-		-destination "name=iPhone 7,OS=10.1" \
+		-destination "name=iPhone X,OS=12.0" \
 		test \
 		| xcpretty -ct
 
@@ -43,7 +43,7 @@ test-tvOS:
 		-project Mapper.xcodeproj \
 		-scheme Mapper \
 		-configuration Release \
-		-destination "platform=tvOS Simulator,name=Apple TV,OS=11.0" \
+		-destination "platform=tvOS Simulator,name=Apple TV,OS=12.0" \
 		test \
 		| xcpretty -ct
 

Package.swift

@@ -1,3 +1,14 @@
+// swift-tools-version:4.2
 import PackageDescription
 
-let package = Package(name: "Mapper")
+let package = Package(
+    name: "Mapper",
+    products: [
+        .library(name: "Mapper", targets: ["Mapper"]),
+    ],
+    targets: [
+        .target(name: "Mapper", path: "Sources"),
+        .testTarget(name: "MapperTests", dependencies: ["Mapper"]),
+    ],
+    swiftLanguageVersions: [.v4, .v4_2]
+)