Adapt to changes in clean task in SBT 0.13.17+
Adapt to changes in clean task in SBT 0.13.17+

We need to customize the new `cleanFilesTask`, rather than the
settings `cleanFiles`.

martijnhoekstra/scala,scala/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,lrytz/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,scala/scala,lrytz/scala,lrytz/scala,martijnhoekstra/scala,martijnhoekstra/scala,scala/scala