Merge branch 'bump-elasticsearch-6' into 'master'

Bump to the latest elasticsearch 6.x

See merge request !1013

base/requirements.txt

 boto3==1.9
 cryptography>=3.1
 Django==3.1.2
-elasticsearch==6.2.0
+elasticsearch==6.8.1
 hiredis==1.1.0
 ijson==2.3
 lxml==4.2.3