Adding EngineCart for testing; upgrading Rspec

This addresses fixes needed after a previous PR merge that broke testing because of Rspec version 3.
You also need to `require 'engine_cart'` in your spec_helper.rb: https://github.com/sul-dlss/spotlight/blob/19de0ba2e19e67f477b3ecc4736f87921a243d34/spec/spec_helper.rb#L6