Callback for get_conda_version.

def _get_conda_version(stdout, stderr):
        """Callback for get_conda_version."""
        # argparse outputs version to stderr in Python < 3.4.
        # http://bugs.python.org/issue18920
        pat = re.compile(r'conda:?\s+(\d+\.\d\S+|unknown)')
        m = pat.match(stderr.decode().strip())
        if m is None:
            m = pat.match(stdout.decode().strip())

        if m is None:
            raise Exception('output did not match: {0}'.format(stderr))

        return m.group(1)