sbt javaopts flags

In OpenJDK 6 and 7, the CMSClassUnloadingEnabled flag does not do anything without also using UseConcMarkSweepGC. This can be confirmed by downloading the OpenJDK source, and viewing hotspot/src/share/vm/runtime/globals.hpp. You can see that by default UseConcMarkSweepGC is set to false. Thus, CMSClassUnloadingEnabled will not do anything without it.