elk: 5.5.2 -> 5.6.1
Release notes:
* https://www.elastic.co/guide/en/elasticsearch/reference/5.6/release-notes-5.6.1.html
* https://www.elastic.co/guide/en/logstash/5.6/logstash-5-6-1.html
* https://www.elastic.co/guide/en/kibana/current/release-notes-5.6.1.html
