Mark the ICBackspaceTextFieldDelegate method as nonobjc to silence the warning

> instance method 'textFieldShouldDelete' nearly matches optional requirement 'textFieldShouldClear' of protocol 'UITextFieldDelegate'

Author: bcylin

Source/TokenField/ICTokenField.swift

@@ -327,7 +327,7 @@ open class ICTokenField: UIView, UITextFieldDelegate, ICBackspaceTextFieldDelega
 
   // MARK: - ICBackspaceTextFieldDelegate
 
-  func textFieldShouldDelete(_ textField: ICBackspaceTextField) -> Bool {
+  @nonobjc func textFieldShouldDelete(_ textField: ICBackspaceTextField) -> Bool {
     if tokens.isEmpty {
       return true
     }