Bump to version 2.8

CHANGELOG.md

-# Master (will become release 2.7)
+# Master (will become release 2.8)
+
+# Release 2.7
 
 - The reference elements have a new method `subEntities`. The result of
   `referenceELement.subEntities(i,codim, c)` is an iterable range

dune.module

 Module: dune-geometry
-Version: 2.7-git
+Version: 2.8-git
 Maintainer: dune-devel@lists.dune-project.org
-Depends: dune-common (>= 2.6)
+Depends: dune-common (>= 2.8)
 Whitespace-Hook: Yes