We are in the process to release Flowpack.ElasticSearch.ContentRepositoryAdaptor 3.0, the main change is the support for recent version of ElasticSearch 2.0 - 2.4.
Warning: This release is NOT compatible with older version of ElasticSearch because the API changes are too big to support older versions.
The support for ElasticSearch 2.0 will be merged next week on master, if your composer.json is wrong your project can break. So please check/upgrade your composer.json.
Until now the documentation of the package instructed to install the package with dependency set to @dev-master, please upgrade your project composer.json to use a stable release. Version 2.0.x of the package supports Elasticsearch 1.x and version 3.0.x will support Elasticsearch 2.x
By the way: We released a minor version of 2.0 with some bugfixes this morning, check https://github.com/Flowpack/Flowpack.ElasticSearch.ContentRepositoryAdaptor/releases/tag/2.0.4
Version 2.0.x will be supported for bugfixes only, new features will be available only on 3.0. You can plan to upgrade your Elasticsearch during the next few months.