Update AtmosphereGwt20.gwt.xml

Removed inherit of the deRPC module file as deRPC was always an experimental feature and has been removed from GWT 2.7, the code does not appear to be using any deRPC features so the suspicion is that the module file was included accidentally.
Merge pull request #158 from rasto68/master

Update AtmosphereGwt20.gwt.xml