[Swift 4] Update Bounded PQ

Author: remlostime

Bounded Priority Queue/BoundedPriorityQueue.playground/Contents.swift

@@ -1,3 +1,8 @@
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
+
 struct Message: Comparable, CustomStringConvertible {
   let name: String
   let priority: Int