Fix/release ci use dune 2.7

The current ci config uses the master branches even for the 2.7 release. This MR updates the ci such that the 2.7 versions of the required dune modules are used. This also updates the compilers to gcc-8 and clang-7, since these are the lowest available compilers in the docker images.

Side note: should the ci changed to only run on pushes to MR? Currently every push to any branch triggers a ci job.