Ok, this problem cropped up in several places. This post (and this one) was informative.
I basically stumbled across the Heroku Buildpack: Clojure. This gives some interesting configuration directions. Specifically, I had to
- set a minimum lein version.
:min-lein-version "2.0.0"
- require JDK 7 and
echo "java.runtime.version=1.7" > system.properties
- added the ring servlet lib:
[ring/ring-servlet "1.2.0-RC1"]
And now everything is copasetic. Hope this helps someone.