Hey guys, I'm a founder of CircleCI. Happy to answer any questions, or check us out and give me feedback!
So the entire backend is clojure. For the devops stuff, we use pallet [1], a clojure library similar to puppet or chef, and stevedore, a clojure DSL for writing bash. We actually have zero bash in the source tree!
The web UI is a fat javascript client using knockout and HAMLcoffee, so the web server only serves a JSON API, all HTML rendering is done client-side.
[1] palletops.com