dbuild should abort when running with JDK7 or later

[rkuhn]

otherwise the scala build fails; in that case it is necessary to delete the scala build directory, or at least I did not manage to find out how to make it build the swing artifacts if it has skipped them once before

[jsuereth]

I think perhaps this should be opened against the scala project. Scala itself should fail if you call targets that cannot be built without JDK6 and you're not running JDK6.

Note: Dbuild can resolve scala from Maven, so you can still run it on JDK7 just fine (I have for sbt-republish).