Update to swift 4

[volkoivan]

Hello! On swift 4 there is an error:
Pods/KNSwitcher/KNSwitcher/KNSwitcher.swift:44:40: Argument of '#selector' refers to instance method 'switcherButtonTouch' that is not exposed to Objective-C

[rursache]

just add @objc before that function and make a pull request