Memory leak with motionNavigationTransitionType in NavigationController

I have memory leak with `motionNavigationTransitionType = .selectBy(presenting: .slide(direction: .left), dismissing: .slide(direction: .right))`

my simple implementation is:

```
import UIKit
import Material

class AppNavigationController: NavigationController {

open override func prepare() {
super.prepare()

isMotionEnabled = true
motionNavigationTransitionType = .selectBy(presenting: .slide(direction: .left), dismissing: .slide(direction: .right))

guard let v = navigationBar as? NavigationBar else { return }
v.dividerColor = Color.grey.lighten2
statusBarStyle = .lightContent

v.backgroundColor = Color.blue.darken2
v.depthPreset = .none

}
}
```
instruments file:

[material_leak.trace.zip](https://github.com/CosmicMind/Material/files/2879419/material_leak.trace.zip)