Invenio-Search is on PyPI. When you install Invenio-Search you must specify the appropriate extras dependency for the version of Elasticsearch you use:
$ # For Elasticsearch 2.x:
$ pip install invenio-search[elasticsearch2]
$ # For Elasticsearch 5.x:
$ pip install invenio-search[elasticsearch5]
$ # For Elasticsearch 6.x:
$ pip install invenio-search[elasticsearch6]