It seems like OPM support for Dune 2.3 is going to be removed sooner
rather than later. Also, all relevant distributions which I'm aware of
seem to ship at least Dune-2.4 packages.
note that this patch switches to Dune 2.4.1 instead of the latest Dune
2.4 release (i.e., 2.4.2) because travis seems to block downloads from
sites it does not know -- in this case dune-project.org -- and the
Dune github mirrors seem to have been abandoned a few months ago and
thus do not feature dune 2.4.2.
f3be13c5af