Elasticsearch dependency should be provided and not compile

Please change the Elasticsearch dependency to provided (and maybe testCompile) instead of compile and runtime.

Why? If you include the plugin in embedded Elasticsearch, and you exclude anything from the Elasticsearch dependency, it is undone by this dependency also including Elasticsearch. Therefore you have to tune both, when really this plugin only needs it as provided (since it runs with Elasticsearch or in Elasticsearch but never on its own).

Closes #48.