rank incorrectly orders ordered categories
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
a = pd.DataFrame(['first', 'second', 'third', 'fourth', 'fifth', 'sixth'], columns=['A'])
a['A'] = a['A'].astype('category', ).cat.set_categories(
    ['first', 'second', 'third', 'fourth', 'fifth', 'sixth'], ordered=True)
a['A'].rank()
# outputs:
# 0    2.0
# 1    4.0
# 2    6.0
# 3    3.0
# 4    1.0
# 5    5.0

```
#### Problem description

rank seems to be ignoring the order of ordered categories.

#### Expected Output
```
0    1.0
1    2.0
2    3.0
3    4.0
4    5.0
5    6.0
```
#### Output of ``pd.show_versions()``

<details>
```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-59-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 34.2.0
Cython: None
numpy: 1.12.0
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
```

</details>
