How to use rolling_std with Pandas 0.23.4?
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({"a": [1, 2, 3,4, 5, 6]})
print("pd.__version__:", pd.__version__)
# pd.__version__: 0.23.4
df['vola'] = pd.rolling_std(df['a'], window=2)
# AttributeError: module 'pandas' has no attribute 'rolling_std'
df['vola'] = df['a'].rolling_std(window=2)
#AttributeError: 'Series' object has no attribute 'rolling_std'

```
#### Problem description

As written in [http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#whatsnew-0180-enhancements](http://pandas.pydata.org/pandas-docs/stable/whatsnew.html#whatsnew-0180-enhancements), the syntax for rolling_std might have changed.

With neither the old nor the new syntax I can calculate the `rolling_std` function.

Is this a bug or do I have to access it differently?