Update gitlab-elasticsearch-indexer to v1.2.0

We need gitlab-elasticsearch-indexer v1.2.0 in GitLab v12.0 . The list of changes in this version is here: gitlab-elasticsearch-indexer!32 (merged)

CE and EE MRs to update the GITLAB_ELASTICSEARCH_INDEXER_VERSION file in gitlab are here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29237 and here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13992 - but that only affects the tests at present.

release-tools!625 (merged) and !3276 (merged) will allow omnibus (and CNG too) to respect that file, but I've not had the chance to complete it in this milestone. So for now, we should just keep the versions in sync.

From an omnibus point of view, the biggest change will be the move from Godep to go.mod - hopefully, that won't break integrations like license_finder. We should make sure of that, though.