Actions.pop always return true

### Version
Tell us which versions you are using:

- react-native-router-flux v4.0.4
- react v16.2.0
- react-native v0.53.3

### Expected behaviour

In RNRF v4.0.0-bate25, Actions.pop return false if the sence is the first page, but 4.0.4 always return true.How do I tell if my stack is currently empty?

### Actual behaviour

As above