Validate that value is a valid version string.

def is_version(value):
    """Validate that value is a valid version string."""
    try:
        value = str(value)
        if not parse_ver('1.4') <= parse_ver(value):
            raise ValueError()
        return value
    except (AttributeError, TypeError, ValueError):
        raise vol.Invalid(
            '{} is not a valid version specifier'.format(value))