Re-download any packages whose status is STALE.

def update(self, quiet=False, prefix='[polyglot_data] '):
    """
    Re-download any packages whose status is STALE.
    """
    self.clear_status_cache()
    for pkg in self.packages():
      if self.status(pkg) == self.STALE:
        self.download(pkg, quiet=quiet, prefix=prefix)