Update Deque up to Swift 4.2.

Author: artFintch

Deque/Deque-Simple.swift

@@ -21,7 +21,7 @@ public struct Deque<T> {
   }
 
   public mutating func enqueueFront(_ element: T) {
-    array.insert(element, atIndex: 0)
+    array.insert(element, at: 0)
   }
 
   public mutating func dequeue() -> T? {

Deque/Deque.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 Deque<T> {
   private var array = [T]()
 

Deque/README.markdown

@@ -23,7 +23,7 @@ public struct Deque<T> {
   }
 
   public mutating func enqueueFront(_ element: T) {
-    array.insert(element, atIndex: 0)
+    array.insert(element, at: 0)
   }
 
   public mutating func dequeue() -> T? {