Fixes for scala 2.11 modularization

Note: The build actually needs to change for the latest Scala 2.11 branch (once modularization is fixed).

This makes use of the fixes here:

https://github.com/typesafehub/distributed-build/pull/47

And fixes our build so this can get merged:

https://github.com/scala/scala/pull/2855

Assuming this is merged first:

https://github.com/adriaanm/scala/pull/7


In particular, this uses another temporary dbuild release (pending review of improvements) and a branch for scala which includes the dbuild-meta.json fixes.   Those will have to point back at scala/scala.git#master before merging.

Also, we removed resolving Scala via ivy for now.   There is no way to construct a complete scala-home in dbuild's sbt rewriting.   It should be innocuous to rebuild scala here, as binary compatibilty should remain in place for the potential few hour difference between build.

Also, if this is going to be used by PR validation, then we'll have to try out the partial support for PR references, see:

https://github.com/typesafehub/distributed-build/issues/35


Reivew by @dragos and @adriaanm