Merge pull request kodecocodes#510 from paulsumit1993/Stack-Swift4

update Stack implementation to Swift 4 with Xcode 9b4

Author: kelvinlauKL

Stack/Stack.playground/Contents.swift

@@ -1,3 +1,9 @@
+
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 /*
   Stack