rolling std results in NaN
#### Code Sample, a copy-pastable example if possible

```python
s = pd.Series([5, 5, 6, 7, 5, 5, 5])
s.rolling(3).std()

```
#### Problem description

The code here is straight from the documentation. 
[https://pandas.pydata.org/pandas-docs/stable/generated/pandas.core.window.Rolling.std.html](url)
The behaviour is inconsistent with previous versions. Rolling std() always results in NaN
but mean, max, etc  work as expected


#### Expected Output
Series with rolling std()


#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.3
pytest: None
pip: 18.0
setuptools: 39.0.1
Cython: None
numpy: 1.15.0
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.5.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.6.0

</details>
