[Version] Bump version to 2.4.1-rc2

dune.module

 # parameters for dune control
 Module: dune-grid-howto
 Maintainer: dune-devel@dune-project.org
-Version: 2.4.1
+Version: 2.4.1-rc2
 Depends: dune-grid (>= 2.4.1)
 # Needed for finiteelements.cc
 Suggests: dune-istl (>= 2.4.1) dune-alugrid