Maven is a tool used to build projects written in Java and some other languages which models a project in an XML structure inside a file usually called pom.xml. One of its strengths is dependency management. Here a short guide.
Both Arquillian and Shrinkwrap group their dependencies in BOMs, Bill Of Materials. You can check available versions in Maven Central.
To add Selenium resources into your project you should have added the Drone extension's BOM
<version>1.0.0.Final</version> -> this is the version in the Drone guide
If you open this project's pom you'll see what are the dependencies it relies on. There you'll find the version for
is 2.20.0, and as I understand that's the problem.
Update the version for the arquillian-drone-bom to 1.1.0.CR2 and the selenium-server will upgrade to 2.24.1 and you should be ready to go.
I hope this *pings* your question.