A few months ago, I started working on some behavior tests for a javascript heavy website. I chose to write the tests in Behat, and wanted to use the Phantom JS driver because it would be faster than running Selenium 2 with a real browser. It also had to run on Circle CI. Once I got this working, several people asked for help getting theirs working, and some were using Travis CI. I threw together this demo repository which has a walkthrough of all the steps you need to follow to get these tools working together on either CI service.
Please let me know if this helps you, or if you have any questions!
Jessica Mauerhan 