Workaround for dbuild's scalaBinaryVersion mishandling.

This is another workaround for typesafehub/dbuild#145. This time we deal with
the problem of Scala version-dependent inclusion of source files to be
compiled. Selection of a source folder is broken because it is based on
scalaBinaryVersion which dbuild overrides. The workaround is to fallback to
"2.11" value for scalaBinaryVersion when the one set in the build is not
included in the known set ("2.9.3, "2.10", "2.11").