reduce version to 2.6.0

dune.module

 Module: dune-fem-dg
-Version: 3.0
+Version: 2.6.0
 Maintainer: dune@mathematik.uni-freiburg.de
 Whitespace-Hook: Yes
-Depends: dune-fem (>=2.5 ) dune-optim ( >= 2.5 )
+Depends: dune-fem
 Sources: dune