59667324 use elasticsearch gem

This change is for https://www.pivotaltracker.com/story/show/59667324

Since [Tire](https://github.com/karmi/retire) is being retired we moved to official [Ruby Elasticsearch client](https://github.com/elasticsearch/elasticsearch-ruby).

As a result we have less complex code. Indexing code isn't so tightly coupled to Sequel model code. We can also run any Elasticsearch query or their combination - no need to wait for Tire add new query types. We now use multi query search endpoint to perform our search and save 4 http requests.

This also makes section search work, sections aren't being found at the moment: https://www.gov.uk/trade-tariff/search?utf8=%E2%9C%93&q=trade_tariff&t=animal+products&button=&day=20&month=1&year=2014&country= doesn't contain Section 1 "Live animals; animal products".

Depends on:

* https://github.com/alphagov/trade-tariff-frontend/pull/114