Heroku comes with Jetty 7.5.4 runner which does not support servlet 3.0. However it is very easy to overcome this as Heroku lets you push your own jetty runner version easily. All you have to do is to download it to a 'server' folder in the root of your project:
mkdir server curl http://repo1.maven.org/maven2/org/mortbay/jetty/jetty-runner/8.1.9.v20130131/jetty-runner-8.1.9.v20130131.jar > server/jetty-runner.jar
And if you like tomcat better you can do this as well. Slick!