Set revision property to string value of integer *value*.

def revision_number(self, value):
        """
        Set revision property to string value of integer *value*.
        """
        if not isinstance(value, int) or value < 1:
            tmpl = "revision property requires positive int, got '%s'"
            raise ValueError(tmpl % value)
        revision = self.get_or_add_revision()
        revision.text = str(value)