downgrade version to 2.6 (current DUNE release)

dune.module

 Module: dune-metagrid
-Version: 3.0.0
+Version: 2.6.0
 Maintainer: dune@mathematik.uni-freiburg.de
-Depends: dune-grid (>= 3.0) dune-alugrid
-Suggests: dune-spgrid (>= 3.0) 
+Depends: dune-grid (>= 2.6) dune-alugrid (>= 2.6)
+Suggests: dune-spgrid (>= 2.6)