Preinstall Scala 2.9.2 for cross-build purpose

Different 2.9.x versions of Scala are binary incompatible.
Happily, Scala patch versions from 2.10.0 on have become binary
compatible, so we won't have to worry about this point in the future.

See https://github.com/sbt/sbt/issues/1439#issuecomment-52964928