title: pip executes the wrong python library versions inside virtual env
body: <p>I create a virtual enviroment with <code>virtualenvwrapper</code> and then I try to install django in it with <code>pip</code>. However I keep getting an error due to a conflict in python versions.</p>
  
  <pre><code>$ mkvirtualenv env
  $ workon env
  $ pip install django
  Downloading/unpacking django
  Cleaning up...
  Exception:
  Traceback (most recent call last):
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/basecommand.py", line 134, in main
      status = self.run(options, args)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/commands/install.py", line 236, in run
      requirement_set.prepare_files(finder, force_root_egg_info=self.bundle, bundle=self.bundle)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/req.py", line 1085, in prepare_files
      url = finder.find_requirement(req_to_install, upgrade=self.upgrade)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/index.py", line 201, in find_requirement
      page = self._get_page(main_index_url, req)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/index.py", line 554, in _get_page
      return HTMLPage.get_page(link, req, cache=self.cache)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/index.py", line 671, in get_page
      resp = urlopen(url)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/download.py", line 176, in __call__
      response = self.get_opener(scheme=scheme).open(url)
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/download.py", line 238, in get_opener
      headers.append(("User-agent", build_user_agent()))
    File "/Users/mingot/virtualenvs/env/lib/python2.7/site-packages/pip/download.py", line 35, in build_user_agent
      _implementation = platform.python_implementation()
    File "/Users/mingot/soft/anaconda/lib/python2.7/platform.py", line 1486, in python_implementation
      return _sys_version()[0]
    File "/Users/mingot/soft/anaconda/lib/python2.7/platform.py", line 1451, in _sys_version
      repr(sys_version))
  ValueError: failed to parse CPython sys.version: '2.7.5 (default, Aug 25 2013, 00:04:04) \n[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)]'
  </code></pre>
  
  <p>In the system, I am running python anaconda:</p>
  
  <pre><code>$ python 
  Python 2.7.5 |Anaconda 1.8.0 (x86_64)| (default, Oct 24 2013, 07:02:20) 
  [GCC 4.0.1 (Apple Inc. build 5493)] on darwin
  </code></pre>
  
  <p>and the <code>$PATH</code> is set to</p>
  
  <pre><code>/Users/mingot/soft/anaconda/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/texbin:/opt/X11/bin
  </code></pre>
  
  <p>while inside the virtual enviroment, the python version is:</p>
  
  <pre><code>(env)$ python
  Python 2.7.5 (default, Aug 25 2013, 00:04:04) 
  [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
  </code></pre>
  
  <p>and the <code>$PATH</code>:</p>
  
  <pre><code>/Users/mingot/virtualenvs/env/bin:/Users/mingot/soft/anaconda/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/texbin:/opt/X11/bin
  </code></pre>
  
  <p>I understand that the problem is that inside the virtual enviroment, while executing the non anaconda python 2.7.5, it is still using <code>platforms.py</code> from anaconda library, causing the crash in the evaluation of the regex as suggested <a href="https://github.com/ContinuumIO/anaconda-issues/issues/33">here</a>. I do not care which python version to use inside the virtual enviroment. Any suggestion on how to tell the python of inside the virtual enviroment the correct <code>platforms.py</code> to use?</p>
  
  <p>Thanks!</p>