Update gitlab-elasticsearch-indexer to v0.2.1

Merged Nick Thomas requested to merge update-gitlab-elasticsearch-indexer into master

I'd like to pick this into %9.5

It fixes a single issue - a runtime panic we are observing on GitLab.com - see https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/issues/8 for more information.

/cc @vlopez @stanhu @smcgivern @psimyn