GemExtensions::Elasticsearch::Model::Indexing::InstanceMethods is no
longer prepended to the proxies, thus the project’s _id is
incorrectly being set as “1” instead of “project_1". This means
joining would fail.

In the past, in config/initializers/elastic_client_setup.rb:

1. ES::Model::Indexing::InstanceMethods is included in
ES::Model::Proxy::InstanceMethodsProxy

2. Our monkey-patch is then prepended to
Elasticsearch::Model::Indexing::InstanceMethods

Since prepending happens later, our monkey patch would not be part of
ES::Model::Proxy::InstanceMethodsProxy's ancestor.

However it did work, by luck, because ES::Git::Model would
later include ES::Model, which would trigger the second
`include` call, and in turn add prepend modules into the ancestors.

But in previous commit, Elasticsearch::Git::Model is removed,
therefore the monkey patch was no longer included.