Merge branch 'feature/update-to-clang-7' into 'master'

Update CI config to clang-7

See merge request !13
3 jobs for master in 7 minutes and 20 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #29916

00:04:24

passed dune:git gcc-6 C++14 #29918

00:05:48

passed dune:git gcc-8 C++17 #29917

00:07:20