download Apache rat if is needed

download Apache rat if is needed

https://github.com/apache/incubator-wayang/blob/4cc0bfdca06dda171b661822daae5fa438d9d475/python/bin/check-license.sh#L22

```sh

#!/bin/bash

################################################################################
##
## Licensed to the Apache Software Foundation (ASF) under one or more
## contributor license agreements. See the NOTICE file distributed with
## this work for additional information regarding copyright ownership.
## The ASF licenses this file to You under the Apache License, Version 2.0
## (the "License"); you may not use this file except in compliance with
## the License. You may obtain a copy of the License at
##
## http://www.apache.org/licenses/LICENSE-2.0
##
## Unless required by applicable law or agreed to in writing, software
## distributed under the License is distributed on an "AS IS" BASIS,
## WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
## See the License for the specific language governing permissions and
## limitations under the License.
##
################################################################################

# TODO download Apache rat if is needed

BASE=$(cd "$(dirname "$0")/.." | pwd)
cd ${BASE}
RAT_HOME=/opt/apache-rat-0.13

java -jar ${RAT_HOME}/apache-rat-0.13.jar -E .rat-excludes -d . | grep "== File:"

```

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