Upgrade elasticsearch-rest-client to 6.4.2
Upgrade elasticsearch-rest-client to 6.4.2

This is a cherry-pick of stable-2.15 recently merged commit f69a822.
This pick assumes that Elasticsearch 6.x.y is still also maintained for
the stable-2.14 branch.

According to the release notes [1] this release does not include
any client fixes. This is just to keep up to date with the latest
version.

[1] https: //www.elastic.co/guide/en/elasticsearch/reference/current/release-notes-6.4.2.html

Change-Id: I3fe434416537625f8963baec85bfabb7c735bed7

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