[fix] update dune version control in constraintsolver to master
The check of the version control using Dune::FMatrixPrecision::set_singular_limit() was removed only in dune-master and not yet in 2.6, therefore the version control is updated so that the ncpflash and pengrobinson tests pass again.
I hope using 2.7 is the correct way of doing this. It worked for me and all material tests pass with dune-2.6 and dune-master.
Edited by Simon Emmert