Revert "Updated Bit Set to Swift 4.2"

This reverts commit 4130f35.

Author: karolms

Bit Set/BitSet.playground/Contents.swift

@@ -1,5 +1,9 @@
 //: Playground - noun: a place where people can play
 
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
 
 // Create a bit set that stores 140 bits
 var bits = BitSet(size: 140)