code: @Override
      public void checkVersion() throws IOException {
          ESVersion esVersion = getVersion();
          if (esVersion.major != compatibleVersion()) {
              throw new RuntimeException("The Elasticsearch client version [" +
                      compatibleVersion() + "] is not compatible with the Elasticsearch cluster version [" +
                      esVersion.toString() + "].");
          }
          if (esVersion.minor < 4) {
              throw new RuntimeException("This version of FSCrawler is not compatible with " +
                      "Elasticsearch version [" +
                      esVersion.toString() + "]. Please upgrade Elasticsearch to at least a 6.4.x version.");
          }
      }
docs: For Elasticsearch 6, we need to make sure we are running at least Elasticsearch 6.4
  @throws IOException when something is wrong while asking the version of the node.