Try making sbt-antlr a source dependency

This would work if sbt-antlr could be compiled by SBT 0.13, but it can't
because of a missing dependency ─ sbt-cross-building 0.8.0 is not
available on 0.13 yet.
Make sbt-antlr a source dependency

I needed to adapt sbt-antlr to 0.13 first.