updating url prop doesn't trigger an update

version : 3.0.1
```javascript
<Router url={props.url}>
  // ...
</Router>
```
In the code above if `props.url` is updated Router doesn't rerender.
Looking at the code it comes from the fact that `state.url` is used in the render method and that it's never updated with `props.url`

I can PR if you want.