Update Package.swift

Updating Package.swift to support Swift 4.2

Author: heyzooi

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:4.0
+// swift-tools-version:4.2
 import PackageDescription
 
 let package = Package(
@@ -17,5 +17,5 @@ let package = Package(
             exclude: ["objc"]
         ),
     ],
-    swiftLanguageVersions: [4]
+    swiftLanguageVersions: [.v4, .v4_2]
 )