The missing dependency is the maven-protoc-plugin, not protobuf-java. This plugin is not available in Maven Central.
According to it’s author you can add a self-hosted repository to the pom.xml:
<pluginRepositories>
<pluginRepository>
<id>protoc-plugin</id>
<url>http://sergei-ivanov.github.com/maven-protoc-plugin/repo/releases/</url>
</pluginRepository>
</pluginRepositories>
But I’m only getting a 404 here. So you can either contact the author or check out the sources and install the plugin manually in your local repository.