Implement Elasticsearch::Model::Response::Pagination::WillPaginate#offset

Fixes #131.

This was a silly mistake on my part, since [will_paginate documents that this method is required](https://github.com/mislav/will_paginate/blob/ca7d45f955066f0a01d500769a8efbb6acc4032c/lib/will_paginate/collection.rb#L7).

I've also submitted https://github.com/mislav/will_paginate/pull/381

I've tested `page_entries_info` in a Rails 4.1.1 app. @johvet Can you please verify this fix in your app?