Fix warning from Xcode 8.3. Fixes #55

Sources/KeychainSwift.swift

@@ -136,7 +136,7 @@ open class KeychainSwift {
   open func get(_ key: String) -> String? {
     if let data = getData(key) {
       
-      if let currentString = NSString(data: data, encoding: String.Encoding.utf8.rawValue) as? String {
+      if let currentString = String(data: data, encoding: .utf8) {
         return currentString
       }