WCS ra_dec_order not working

When using `all_pix2world` with `ra_dec_order=True` I get:

```python
  File ".../site-packages/astropy/wcs/wcs.py", line 1099, in all_pix2world
    self._all_pix2world, 'output', *args, **kwargs)
  File ".../site-packages/astropy/wcs/wcs.py", line 1088, in _array_converter
    return _return_list_of_arrays(axes, origin)
  File ".../site-packages/astropy/wcs/wcs.py", line 1046, in _return_list_of_arrays
    output = self._normalize_sky_output(output)
AttributeError: 'WCS' object has no attribute '_normalize_sky_output'
```

Which makes sense as there is no method named `_normalize_sky_output`, only `_normalize_sky`.

I use astropy 0.4.