Update swiftLanguageVersions to swift-tools-version:4.2 (#2736)

This API changed from taking [Int] to [SwiftVersion] in PackageDescription API Version 4.2. For more information, see https://github.com/apple/swift-package-manager/blob/master/Documentation/PackageDescriptionV4_2.md#swift-language-version.

Package.swift

@@ -37,5 +37,5 @@ let package = Package(
             name: "Alamofire",
             path: "Source")
     ],
-    swiftLanguageVersions: [3, 4]
+    swiftLanguageVersions: [.v3, .v4]
 )