Fix ssl_wrap_socket AttributeError on python2

Importing requests on python2 raises an AttributeError in pyopenssl.py

    Python 2.7.5 (default, Sep  6 2013, 09:55:21) 
    [GCC 4.8.1 20130725 (prerelease)] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import requests
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python2.7/site-packages/requests/__init__.py", line 53, in <module>
        from .packages.urllib3.contrib import pyopenssl
      File "/usr/lib/python2.7/site-packages/requests/packages/urllib3/contrib/pyopenssl.py", line 55, in <module>
        orig_connectionpool_ssl_wrap_socket = connectionpool.ssl_wrap_socket
    AttributeError: 'module' object has no attribute 'ssl_wrap_socket'