Update ES to 6.8.12

[stsewd]

We need this before updating to 7.x. No changes were required, everything just works :)

We need to merge readthedocs/common#72 too.

ref #5620

We need to update to this version in the admin panel of ES. We don't need to reindex for this operation

[humitos]

I think this is OK. Should we update ES on production as well together with the update on Travis and local development environment?

[humitos]

Do we need to follow this guide on production? https://www.elastic.co/guide/en/elasticsearch/reference/6.8/rolling-upgrades.html

[stsewd]

@humitos there is a button in the ES admin page, so we don't need to follow that guide. And yeah, looks like we could update this outside the deploy if we want.