Gaps show in list occasionally when dragging

Sometimes gaps show up in the list when dragging as mentioned in https://github.com/deanmcpherson/react-native-sortable-listview/issues/2

I believe the fix for the gaps is to update `onPanRepsonderRelease()`'s last state assignment from
```
this.state.active = false;
this.state.hovering = false;
```
to
```
if (this.props._legacySupport) {
this.state.active = false;
this.state.hovering = false;
} else {
this.setState({
active: false,
hovering: false
});
}
```
to match the assignment further up to trigger a refresh.

I can submit a PR but I've not had a chance to test this across iOS/Android versions yet.