Merge branch 'bump-version-to-2.6' into 'master'

Change module version number to 2.6-git

See merge request !139

dune.module

 Module: dune-grid
-Version: 3.0-git
+Version: 2.6-git
 Maintainer: dune-devel@dune-project.org
-Depends: dune-geometry (>= 3.0)
+Depends: dune-geometry (>= 2.6)
 Suggests: dune-uggrid
 Whitespace-Hook: Yes