logstash: Upgrade to version 6.4.2

The version 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