df.rank() has different behavior for python2 and python3
#### Code Sample, a copy-pastable example if possible


python3
```python
In [1]: import pandas as pd

In [2]: pandas_df = pd.DataFrame({
   ...:     'col1': [1, 2, 3, 4],
   ...:     'col2': [4, 5, 6, 7],
   ...:     'col3': [8.0, 9.4, 10.1, 11.3],
   ...:     'col4': ['a', 'b', 'c', 'd']})
   ...:

In [3]: pandas_df.rank(axis=1)
Out[3]:
   col1  col2  col3
0   1.0   2.0   3.0
1   1.0   2.0   3.0
2   1.0   2.0   3.0
3   1.0   2.0   3.0
```

python2
```python
In [1]: import pandas as pd

In [2]: pandas_df = pd.DataFrame({
   ...:     'col1': [1, 2, 3, 4],
   ...:     'col2': [4, 5, 6, 7],
   ...:     'col3': [8.0, 9.4, 10.1, 11.3],
   ...:     'col4': ['a', 'b', 'c', 'd']})
   ...:

In [3]: pandas_df.rank(axis=1)
Out[3]:
  col1  col2  col3  col4
0   1.0   2.0   3.0   4.0
1   1.0   2.0   3.0   4.0
2   1.0   2.0   3.0   4.0
3   1.0   2.0   3.0   4.0
```


#### Problem description

Running pandas on python2 produces different output from running on python3 for df.rank on mixed dtype dataframes. The expected output should be to receive the python3 version of the output, as strings and integers should not be automatically compared, similarly to how pandas handles quantile/describe. 

#### Expected Output
```python
In [1]: import pandas as pd

In [2]: pandas_df = pd.DataFrame({
   ...:     'col1': [1, 2, 3, 4],
   ...:     'col2': [4, 5, 6, 7],
   ...:     'col3': [8.0, 9.4, 10.1, 11.3],
   ...:     'col4': ['a', 'b', 'c', 'd']})
   ...:

In [3]: pandas_df.rank(axis=1)
Out[3]:
   col1  col2  col3
0   1.0   2.0   3.0
1   1.0   2.0   3.0
2   1.0   2.0   3.0
3   1.0   2.0   3.0
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-43-Microsoft
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.2.1
pip: 10.0.1
setuptools: 36.5.0.post20170921
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.7.0
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>