Ruby framework for Selenium webdriver

I am working on a webapplication (Ruby on rails) as a QA (Mainly blackbox testing). As the application grown bigger, we need to automate the testing. Selenium webdriver as we found as the de facto tool. But since entire dev team is using Ruby and no one really has time to learn Java, we decided to go for Ruby and webdriver combination. Here, I am quite newbie to automation test framework - not really sure about which framework to go with as far as ruby and webdriver is concerned. I also need to train a junior QA, I am looking for something relatively easy to start with. Please guide. Thanks