Change Result type condition to work in Xcode 10.1

Sources/Utility/Result.swift

@@ -26,7 +26,9 @@
 
 import Foundation
 
-#if swift(<5)
+#if swift(>=4.3)
+/// Result type already built-in
+#else
 /// A value that represents either a success or failure, capturing associated
 /// values in both cases.
 public enum Result<Success, Failure> {