no support for reverse_nested aggregations

[jameslamb]

Per discussion in #58 ... es_search does not currently support reverse_nested aggregations. Well, technically it might support them but we have no tests around that.

To be honest I have no idea how those aggregations work, but would love for someone knowledgable to take a run at this issue. Reference on them:

https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-reverse-nested-aggregation.html

Requirements for closing this issue:

1. Add code to our integration tests that create a structure in our test ES cluster that even allows this type of query (might require figuring out which versions of Elasticsearch actually support reverse_nested stuff)
2. Create tests that confirm expected results for queries with reverse_nested stuff in them

[jameslamb]

A fix for this is not going to go out on the next release.