You need to know the version of Jetty that uses the application based on the Restlet environment. All I know is that the version of Restlet you are using is 2.1 from the pom.xml file in the project.
use the --version flag:
java -jar start.jar --version
If you add -X to the build command line, it should show how maven resolves the dependencies, providing you with a complete chain of dependencies, including which version of the pier you can pull out.
IDE , pom.xml Eclipse m2eclipse, .