Update Run-Length Encoding to Swift 4

Updated to Swift 4 and Xcode 9.0b4. No changes necessary.

Author: bobdel

Run-Length Encoding/RLE.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
+
 import Foundation
 
 let originalString = "aaaaabbbcdeeeeeeef"