`value_counts` does not respect ordered categoricals
#### Code Sample, a copy-pastable example if possible
```python
import pandas as pd

cat = pd.Series(pd.Categorical(
                ['a', 'c', 'a', 'b', 'a'],
                categories=['a', 'b', 'c'],
                ordered=True))

cat.value_counts()
```

#### Problem description
This outputs the value counts in the order 'a', 'c', 'b' when the categorical is ordered 'a', 'b', 'c'. `value_counts` should respect ordered categoricals and display them appropriately.

#### Expected Output
`value_counts` outputs in the specified order: 'a', 'b', 'c'

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.6.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.0
numpy            : 1.17.0
pytz             : 2019.1
dateutil         : 2.7.5
pip              : 19.2.1
setuptools       : 41.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.2.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.0
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
</details>
