mvn appengine:<anything> do not work: no plugin found

[hgabreu]

I followed the simple instructions on the readme: clone; cd; mvn appengine:devserver
Then I get a build failure with the following error:
[ERROR] No plugin found for prefix 'appengine' in the current project and in the plugin groups [org.apache.maven.plugins, org.codehaus.mojo] available from the repositories [local, central] -> [Help 1]

Is there anything I'm missing or this is indeed not working anymore?