add missing impl of new abstract method coming with atmosphere-2.x

important: only merge this change in conjunction with the upgrade to atmosphere-2.x

after upgrading to atmosphere-2.x _and_(!) a grails clean, this missing method impl (coming from abstract onClose(AtmosphereResourceEvent) in AtmosphereResourceEventListener) generates a compilation error
Merge pull request #46 from zyro23/patch-3

add missing impl of new abstract method coming with atmosphere-2.x