Instal script needs to be updated

• It seems that the combination of dune-subgrid(2.6-1) and dune(git) does not compile: coupling free flow and porous media flow exercise. There are also problems in dumux test which use dune-subgrid.
• However, there is no problem related to dune-subgrid(2.6-1) and dune(2.7). Thus, The versions of dumux(3.0) and dune(2.6) in install script on the website needs to be updated after dumux 3.2 release.