Bump to the latest elasticsearch 6.x

We are using elasticsearch 6.2.0, which only allows urllib3<=1.23. Since we are now at urllib3 1.25, this breaks bumps on requests and responses: https://gitlab.com/arkindex/backend/-/jobs/775737878#L155

This has however been fixed in version 6.4.0: https://github.com/elastic/elasticsearch-py/commit/4352e56174b77560d2f86801cb1ad32440bb2d32#diff-2eeaed663bd0d25b7e608891384b7298R15

Since upgrading to ES 7 requires much more work, we can in the meantime upgrade to the latest version of elasticsearch to avoid this urllib3 issue and bump all the other things.