With-apollo: use reduxRootKey

@ads1018 the Apollo doc is a bit confusing (https://github.com/apollographql/react-docs/issues/123) but I think `apollo` shouldn't be hard-coded, instead should use something along the lines of:

```js
  const initialState = {
    ...state,
    [client.reduxRootKey]: {
      data: client.store.getState()[client.reduxRootKey].data
    }
  };
```