(Re)-deprecate order keyword in factorize
In https://github.com/pandas-dev/pandas/pull/6930 we deprecated `order`.

As of 0.18.1, the keyword was no-longer documented, and doesn't raise, but wasn't actually removed.

```python
In [3]: pandas.factorize([0, 1], order=True)
Out[3]: (array([0, 1]), array([0, 1]))

In [4]: pandas.__version__
Out[4]: '0.18.1'
```