Allows --pex-args to take an argument
Allows --pex-args to take an argument

Prevents this error:

    $ python setup.py bdist_pex --pex-args='-r gunicorn-req.txt -r ipython-req.txt'
    usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
       or: setup.py --help [cmd1 cmd2 ...]
       or: setup.py --help-commands
       or: setup.py cmd --help

    error: option --pex-args must not have an argument

mzdanieltest/pex,pantsbuild/pex,pombredanne/pex,jsirois/pex,URXtech/pex,rouge8/pex,jamesbroadhead/pex,pombredanne/pex,mzdanieltest/pex,snyaggarwal/pex,jsirois/pex,sixninetynine/pex,sixninetynine/pex,kwlzn/pex,URXtech/pex,pantsbuild/pex,kwlzn/pex,jamesbroadhead/pex,rouge8/pex,snyaggarwal/pex