update to use new Elasticsearch API

Hello!

I've been having issues installing and carton-ing my dependencies, and I think it's because of ElasticSearchX::UniqueKey.  I use Elastic::Model, which depends on both Elasticsearch and ElasticSearchX::UniqueKey.  UniqueKey depends on ElasticSearch (the old api) and since I'm on a case-insensitive file-system, I think it's messing with getting Elasticsearch installed. 

This branch converts UniqueKey to use the new Elasticsearch API.  It now passes all of its tests, and no longer depends on the old module.  Please let me know if you'd like me to make any changes and thanks for all your effort on this family of modules!

Cheers,
Fitz

P.S. I've got another branch in my repo that gets the tests passing by subbing in Elasticsearch::Compat, but that still required minor changes to the module.  See the branch called new_api_via_compat.