unnecessary checkInitialRef on child initial or substate

https://github.com/jbeard4/SCION-CORE/blob/07e2a1e9dc7d2437933f97cb916e9f098647b5b7/lib/scion.js#L187

```
state.initialRef = initialChildren.length ? initialChildren[0] : state.states[0];
checkInitialRef(state);
```

When initialRef is set to the first &lt;initial> or child &lt;state> the reference is guaranteed to be valid given that the current state is a composite.
