Only use duplicate typealias in Swift versions lower than 3.1

Fixes #39.

Author: tonyarnold

Sources/ExtendedDiff.swift

@@ -9,7 +9,11 @@
  */
 public struct ExtendedDiff: DiffProtocol {
 
+    #if swift(>=3.1)
+    // The duplicate typealias is not required in Swift 3.1
+    #else
     public typealias Index = Int
+    #endif
 
     public enum Element {
         case insert(at: Int)

Sources/NestedDiff.swift

@@ -1,7 +1,11 @@
 
 public struct NestedDiff: DiffProtocol {
 
+    #if swift(>=3.1)
+    // The duplicate typealias is not required in Swift 3.1
+    #else
     public typealias Index = Int
+    #endif
 
     public enum Element {
         case deleteSection(Int)

Sources/NestedExtendedDiff.swift

@@ -1,7 +1,11 @@
 
 public struct NestedExtendedDiff: DiffProtocol {
 
+    #if swift(>=3.1)
+    // The duplicate typealias is not required in Swift 3.1
+    #else
     public typealias Index = Int
+    #endif
 
     public enum Element {
         case deleteSection(Int)