add opm-core to the ewoms prerequisites
remove the dune-istl from the list of opm-material prerequisites

the only reason dune-istl was required was that the spline class used
ISTL's tridiagonal matrix. Since the spline class moved into the core
(along with a more capable tridiagonal matrix), ISTL is no longer
required by opm-material.
Merge pull request #387 from andlaus/change_material_and_ewoms_prerequisits

Change material and ewoms prerequisites