Replace 'Generator' with 'Iterator'

In the latest Swift 3.1 snapshots Argo ceases to build with: `error:
'Generator' has been renamed to 'Iterator'`.

I think this is safe for all of Swift 3.x though.

Sources/Argo/Types/StandardTypes.swift

@@ -194,9 +194,9 @@ public extension Collection where Iterator.Element: Decodable, Iterator.Element
 
     - returns: A decoded array of values
   */
-  static func decode(_ json: JSON) -> Decoded<[Generator.Element]> {
+  static func decode(_ json: JSON) -> Decoded<[Iterator.Element]> {
     switch json {
-    case let .array(a): return sequence(a.map(Generator.Element.decode))
+    case let .array(a): return sequence(a.map(Iterator.Element.decode))
     default: return .typeMismatch(expected: "Array", actual: json)
     }
   }