add support for win32 platform python

Installation of emacs-jedi fails in win32 platform.
The virtualenv installs pip to `$(ENV)/Scripts` when `sys.platform == 'win32'` instead of `$(ENV)/bin`.
That is why emacs-jedi make fails in win32 platform, and this pull request fixes this problem.

The path behavior of virtualenv described in `path_location` function in `virtualenv.py`, see https://github.com/pypa/virtualenv/blob/develop/virtualenv.py#L924 for more information.