- update to 1.7.6
https://www.elastic.co/downloads/past-releases/elasticsearch-1-7-6

Note that 1.7.x is EOL since a month, but still - please don't upgrade
to another branch.

The MediaWiki 1.27.x LTS release still needs 1.7.x :-/ - at least this
is under discussion on https://phabricator.wikimedia.org/T146636