Updated SetCover to Swift 4

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

Author: bobdel

Set Cover (Unweighted)/SetCover.playground/Contents.swift

@@ -1,3 +1,10 @@
+// SetCover
+
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 let universe1 = Set(1...7)
 let array1 = randomArrayOfSets(covering: universe1)
 let cover1 = universe1.cover(within: array1)