Fix Error using Swift 4.0 and Xcode 9 (#15)

Xcode 9 now requires explicit `@objc` marking for dynamic functions.

Author: Miralem Cebic

ReSwiftRecorder/UI/StateHistorySliderView.swift

@@ -35,6 +35,7 @@ class StateHistorySliderView: UIView {
         fatalError("init(coder:) has not been implemented")
     }
 
+    @objc
     dynamic func sliderValueChanged() {
         stateSelectionCallback?(Int(slider.value))
     }