Elasticsearch: Ruby indexer no longer handles blob language

We removed Linguist from CE (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21008), but the Elasticsearch Ruby indexer still depends on determining the language here: https://gitlab.com/gitlab-org/gitlab-ee/blob/b958e271cdf9fe46bbb07b65dd28f0c6ba12dbd1/ee/lib/elasticsearch/git/repository.rb#L154.

To fix a CE->EE merge, we set the value to nil in 5296b91e, and removed the require in 8b919b8c.

The Golang Elasticsearch indexer has its own implementation to determine the language type. When the Ruby indexer goes away, then this issue may not be needed.

However, if we do want to fix the Ruby indexer, we would need to fix this, possibly by:

1. Bringing back github-linguist only for Elasticsearch users
2. Changing Gitaly to return the language type with the blob.

/cc: @mdelaossa, @nick.thomas