[INSTALL.cmake]

Fix minimum CMake version, it's 2.8.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9278 2fb0f335-1f38-0410-981e-8018bf24f1b0

INSTALL.cmake

 Why CMake
 =========
 
-You can use CMake 2.6 or higher as alternative to the build system 
+You can use CMake 2.8 or higher as alternative to the build system 
 provided by DUNE. CMake is included in most GNU/Linux distributions 
 or can be downloaded at www.cmake.org. Using CMake has several 
 advantages compared to autotools: