Merge pull request #512 from paulsumit1993/gcd-swift4

update gcd implementation to Swift 4 with Xcode 9b4

Author: kelvinlauKL

GCD/GCD.playground/Contents.swift

@@ -1,5 +1,10 @@
 //: Playground - noun: a place where people can play
 
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 // Recursive version
 func gcd(_ a: Int, _ b: Int) -> Int {
   let r = a % b