logstash: Pin version to current 6.4.2

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

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Migrated from D544 (view on Phabricator)