Travis-CI will give you, for free, one single-threaded build service for all your students. This works very well up to the few minutes prior to a submission deadline, when all the students push their code all at once. I ended up with backlogs that took 3+ hours to clear. You should make sure your students understand they can run all the same tests locally, rather than having them rely on Travis-CI.
I bugged the Travis-CI people about getting me more concurrency, and they suggested that students could host the repos in their personal Github projects (e.g., github.com/danwallach rather than github.com/RiceComp215). I decided against this because it would be a bigger headache for the grading process. I want every repo all in one place.