compatibility with ElasticSearch 6.x

[danie-dejager]

The software does not seem to allow archiving if ElasticSearch 6.x is being used.

I upgraded Graylog to use EleasticSearch 6 when I upgraded to Graylog 2.5.1. My config for archiving does now not see the indices anymore.

# /bin/graylog-archiver -c graylog_arch.conf
2019-01-28 11:11:14,505 [INFO] Indices: ['graylog_0']
2019-01-28 11:11:14,508 [INFO] Archiving []
#

Nothing happens after running the command.