Update Ring Buffer to Swift 4.2

Author: BNTR

Ring Buffer/RingBuffer.playground/Contents.swift

@@ -1,10 +1,5 @@
 //: Playground - noun: a place where people can play
 
-// last checked with Xcode 9.0b4
-#if swift(>=4.0)
-print("Hello, Swift 4!")
-#endif
-
 public struct RingBuffer<T> {
   fileprivate var array: [T?]
   fileprivate var readIndex = 0