Python virtualenv не находит атрибут 'mac_ver' при внезапной установке нового env

Question

Этим утром я столкнулся с проблемой Virtualenv на моем MacBook Pro в Yosemite (10.10.3):

  $ virtualenv ENV
  New python executable in ENV/bin/python2.7
  Also creating executable in ENV/bin/python
  Installing setuptools, pip, wheel...
    Complete output from command /Users/USER/Docu...jp/ENV/bin/python2.7 -c "import sys, pip; sys...d\"] + sys.argv[1:]))" setuptools pip wheel:
    Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/__init__.py", line 13, in <module>
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/utils/__init__.py", line 23, in <module>
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/_vendor/__init__.py", line 72, in load_module
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/_vendor/pkg_resources/__init__.py", line 967, in <module>
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/_vendor/pkg_resources/__init__.py", line 970, in Environment
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/_vendor/pkg_resources/__init__.py", line 260, in get_supported_platform
    File "/usr/local/lib/python2.7/site-packages/virtualenv_support/pip-7.0.3-py2.py3-none-any.whl/pip/_vendor/pkg_resources/__init__.py", line 424, in _macosx_vers
  AttributeError: 'module' object has no attribute 'mac_ver'
  ----------------------------------------
  ...Installing setuptools, pip, wheel...done.
  Traceback (most recent call last):
    File "/usr/local/bin/virtualenv-3.4", line 9, in <module>
      load_entry_point('virtualenv==13.0.3', 'console_scripts', 'virtualenv-3.4')()
    File "/usr/local/lib/python2.7/site-packages/virtualenv.py", line 832, in main
      symlink=options.symlink)
    File "/usr/local/lib/python2.7/site-packages/virtualenv.py", line 1004, in create_environment
      install_wheel(to_install, py_executable, search_dirs)
    File "/usr/local/lib/python2.7/site-packages/virtualenv.py", line 969, in install_wheel
      'PIP_NO_INDEX': '1'
    File "/usr/local/lib/python2.7/site-packages/virtualenv.py", line 910, in call_subprocess
      % (cmd_desc, proc.returncode))
  OSError: Command /Users/USER/Docu...jp/ENV/bin/python2.7 -c "import sys, pip; sys...d\"] + sys.argv[1:]))" setuptools pip wheel failed with error code 1

Я использую virtualenv, установленный вместе с brew:

  $ which virtualenv
  /usr/local/bin/virtualenv

Обратите внимание, я использую рыбную скорлупу, но пробовал при bash, и у меня похожие симптомы. Удаление python, pyenv и virtualenv и запуск заново (с homebrew) не решили проблему. В чем может быть проблема? Я пытаюсь.

Спасибо за помощь