Drop stderr in pip --version

In pip_version fact drop stderr when calling `pip --version` command. Some old versions of `pip` doesn't support `--version` argument. E.g. on Debian 6 (python-pip_0.7.2-1):

    # pip --version
    Usage: pip COMMAND [OPTIONS]
    
    pip: error: no such option: --version

and facter then complains:

    # facter -p >/dev/null
    Usage: pip COMMAND [OPTIONS]

    pip: error: no such option: --version