logstash: Upgrade to version 6.4.2

The version initially required (6.3.2) was not pinned so it got
upgraded to 6.4.2. The recent changes included fixes at least one
wrong behavior in the worker's logs (regarding indexed field) so that
sounds reasonable to keep it.

[1] https://www.elastic.co/guide/en/logstash/current/logstash-6-4-2.html