Travis pipeline fails at building state

Travis pipeline fails at building state

See https://travis-ci.org/SoilRos/dune-copasi/jobs/595618152

For linux it fails at this line:

home/travis/build/SoilRos/dune-grid/dune/grid/io/file/vtk/b64enc.hh:60:42: error: missing terminating ' character
   60 |       const unsigned A = (txt[0] & 0b1111'1100) >> 2;
      |                                          ^~~~~~~~~~~~

For mac the error looks like this:

/Users/travis/build/SoilRos/dune-copasi/dune/copasi/enum.hh:4:15: warning: nested namespace definition is a C++17 extension; define each namespace separately [-Wc++17-extensions]
namespace Dune::Copasi {
              ^~~~~~~~
               { namespace Copasi
In file included from /Users/travis/build/SoilRos/dune-copasi/lib/model_multidomain_diffusion_reaction.cc:5:
In file included from /Users/travis/build/SoilRos/dune-copasi/dune/copasi/model_multidomain_diffusion_reaction.hh:4:
In file included from /Users/travis/build/SoilRos/dune-copasi/dune/copasi/concepts/grid.hh:4:
In file included from /Users/travis/build/SoilRos/dune-functions/dune/functions/common/functionconcepts.hh:6:
In file included from /Users/travis/build/SoilRos/dune-common/dune/common/typelist.hh:10:
In file included from /Users/travis/build/SoilRos/dune-common/dune/common/std/type_traits.hh:7:
/Users/travis/build/SoilRos/dune-common/dune/common/typetraits.hh:356:28: error: no template named 'add_lvalue_reference_t' in namespace 'std'; did you mean 'add_lvalue_reference'?
      ++(std::declval<std::add_lvalue_reference_t<decltype(std::declval<T>().begin())>>()),
                      ~~~~~^