beautifulsoup fails to install in requirements_all.txt
#### Code Sample, a copy-pastable example if possible

```shell
(root) dchen@longclaw [~/git/fork/pandas]  (master) 
[15:57:39] $ conda create -n pandas_dev_2 python=3 --file ci/requirements_dev.txt
Fetching package metadata ...........
Solving package specifications: .

Package plan for installation in environment /home/dchen/anaconda3/envs/pandas_dev_2:

The following NEW packages will be INSTALLED:

    coverage:        4.4.1-py36_0 
    cython:          0.25.2-py36_0
    flake8:          3.3.0-py36_0 
    mccabe:          0.6.1-py36_0 
    mkl:             2017.0.3-0   
    numpy:           1.13.1-py36_0
    openssl:         1.0.2l-0     
    pip:             9.0.1-py36_1 
    py:              1.4.34-py36_0
    pycodestyle:     2.3.1-py36_0 
    pyflakes:        1.5.0-py36_0 
    pytest:          3.1.2-py36_0 
    pytest-cov:      2.3.1-py36_0 
    python:          3.6.1-2      
    python-dateutil: 2.6.0-py36_0 
    pytz:            2017.2-py36_0
    readline:        6.2-2        
    setuptools:      27.2.0-py36_0
    six:             1.10.0-py36_0
    sqlite:          3.13.0-0     
    tk:              8.5.18-0     
    wheel:           0.29.0-py36_0
    xz:              5.2.2-1      
    zlib:            1.2.8-3      

Proceed ([y]/n)? 

#
# To activate this environment, use:
# > source activate pandas_dev_2
#
# To deactivate an active environment, use:
# > source deactivate
#

(root) dchen@longclaw [~/git/fork/pandas]  (master) 
[15:57:49] $ source activate pandas_dev_2
(pandas_dev_2) dchen@longclaw [~/git/fork/pandas]  (master) 
[15:58:08] $ conda install -n pandas_dev_2 -c pandas --file ci/requirements_all.txt
Fetching package metadata .............

PackageNotFoundError: Package missing in current linux-64 channels: 
  - nbsphinx

Close matches found; did you mean one of these?

    nbsphinx: sphinx

(pandas_dev_2) dchen@longclaw [~/git/fork/pandas]  (master) 
[15:58:56] $ conda install -n pandas_dev_2 -c conda-forge --file ci/requirements_all.txt
Fetching package metadata .............
Solving package specifications: .

UnsatisfiableError: The following specifications were found to be in conflict:
  - beautiful-soup -> python 2.6*
  - python 3.6*
Use "conda info <package>" to see the dependencies for each package.
```
#### Problem description

This will prevent people from contributing since the environment will not be created correctly.
Current Hack is to remove `beautifulsoup` from `ci/requirements_all.txt`
and then manually installing it `conda install -c conda-forge beautifulsoup4`

#### Expected Output

No errors, everything installs.

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

<details>

INSTALLED VERSIONS
------------------
commit: 96168ef698ac8bbccba251258ee66958359b11bf
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.11.9-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0.dev+243.g96168ef69.dirty
pytest: 3.1.3
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.5.0a2
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.8.0
bs4: None
html5lib: 0.9999999
sqlalchemy: 1.1.11
pymysql: 0.7.9.None
psycopg2: None
jinja2: 2.9.5
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
