Issues with requests != 2.12

In #58 I see you added `requests != 2.12` to the requirements of `adal`. This is sadly not a workable restriction in practice due to the way pip currently does version dependency resolution. To illustrate, consider this simple Python package setup.py:



```python

from setuptools import setup, find_packages



version = '0.1'



setup(

    name='depfail',

    version=version,

    packages=find_packages(),

    entry_points={

        'console_scripts': [

            'run-me = depfail:fail',

        ]

    },

    install_requires=[

        'requests[security]',

        'adal',

    ],

)

```



We'll also have a `depfail/__init__.py` that looks like this:



```python

def fail():

    print("No failure!")

```



This package has two dependencies, `requests[security]` and `adal`. You might rationally think that pip would recursively look through dependencies to build a full set of requirements and then use something like a SAT solver to satisfy the constraints imposed on individual packages (and, of course, fail if it can't satisfy them). Unfortunately, that is not the case. In reality here's what your virtualenv will look like if you do `pip install .`:



```

adal (0.4.3)

cffi (1.9.1)

cryptography (1.6)

depfail (0.1)

enum34 (1.1.6)

idna (2.1)

ipaddress (1.0.17)

pip (9.0.1)

pyasn1 (0.1.9)

pycparser (2.17)

PyJWT (1.4.2)

pyOpenSSL (16.2.0)

python-dateutil (2.6.0)

requests (2.12.3)

setuptools (30.2.0)

six (1.10.0)

wheel (0.30.0a0)

```



As you can see, requests 2.12.3 is installed despite the explicit `!= 2.12`. Of course if we do a `python -c 'import depfail;depfail.fail()'` we get `No failure!`. So what's the problem?



Well, when you define a console_script it uses setuptools entry points to invoke your script. The installed script is available in your PATH and looks something like this:



```

# EASY-INSTALL-ENTRY-SCRIPT: 'depfail==0.1','console_scripts','run-me'

__requires__ = 'depfail==0.1'

import re

import sys

from pkg_resources import load_entry_point



if __name__ == '__main__':

    sys.argv[0] = re.sub(r'(-script\.pyw?|\.exe)?$', '', sys.argv[0])

    sys.exit(

        load_entry_point('depfail==0.1', 'console_scripts', 'run-me')()

    )

```



`load_entry_point` is a bit more conscientious about whether or not dependencies are satisfied so it proceeds to check to see if adal's requirements are met and...



```

pkg_resources.ContextualVersionConflict: (requests 2.12.3 (/path/to/lib/python2.7/site-packages/requests-2.12.3-py2.7.egg), Requirement.parse('requests!=2.12.*,>=2.0.0'), set(['adal']))

```



This interaction means that `adal` can't effectively block requests 2.12 for users who have requests listed as a dependency that is processed before `adal` (a presumably common scenario). Additionally, this directive causes major breakage for users who invoke their application via console scripts (not an uncommon path).

[]