Compatibility with old webpack

This doesn't work for older versions of webpack. For compatibility, can you consider changing:

```
const isEntryChunk = (chunk: Object) => {
return chunk.hasRuntime() && chunk.isInitial();
};
```

to:

```
const isEntryChunk = (chunk: Object) => {
return ((chunk.hasRuntime && chunk.hasRuntime()) || chunk.entry) && ((chunk.isInitial && chunk.isInitial()) || chunk.initial);
};
```