Added support for bulk update

    # update mapping
    index = Tire::Index.new(Contact.tire.index_name)
    puts index.mapping(
      Contact.tire.document_type,
      properties: {
        updated_at: {
          type: 'date',
          format: 'date_time_no_millis'
        }
      }
    )

    # add data
    Contact.find_in_batches do |batch|
      documents = batch.map do |contact|
        {
          id: contact.id,
          type: Contact.tire.document_type,
          _routing: contact.account_id,
          doc: ContactSearchSerializer.new(contact, only: :updated_at).as_json
        }
      end
      puts index.bulk_update(documents)
    end

Closes #853