Jump when arriving on custom navbar

### Version
- react-native-router-flux v4.0.6
- react v16.5.0
- react-native v0.57.1

### Expected behaviour
No transition when going to screen with custom nav bar as it is the case with default navbar.

### Actual behaviour
There is some kind of "jump" transition when arriving to custom navbar, either coming from default navbar or from another custom navbar.

### Steps to reproduce
From examples/react-native, go to custom nav bar and then to another custom navbar, you can see the "jump" I'm talking of. With standard navbars, there is no transition.