Update ELK images
elasticsearch:
- https://github.com/docker-library/elasticsearch/commit/c9aa8ee: Update to 6.8.1
- https://github.com/docker-library/elasticsearch/commit/8ee2df9: Update generated README
logstash:
- https://github.com/docker-library/logstash/commit/a3e3579: Update to 6.8.1
- https://github.com/docker-library/logstash/commit/4f086b1: Update generated README
kibana:
- https://github.com/docker-library/kibana/commit/539ce0a: Update to 6.8.1
- https://github.com/docker-library/kibana/commit/5651d02: Update generated README
Docker Library Bot