Make sure Skybar is built first:

cd skybar
./gradlew

Then run the demo:

cd skybar-demo
mvn clean package
java -javaagent:../skybar/build/libs/skybar-1.0-SNAPSHOT-all.jar \
  -Dskybar.includedPackages=com/skybar/demo \
  -jar target/skybar-demo-1.0-SNAPSHOT-jetty-console.war --headless

or using Maven:

mvn jetty:run-forked