TravisCI: Try manually setting compiler
TravisCI: Try manually setting compiler

While the "compiler:" flag can be used in .travis.yml, this appears to hard-code environment variables CC and CXX to the "default" compiler paths, i.e. g++ and clang++, rather than the paths to the newer versions of those compilers actually installed. As a consequence, while TravisCI reports the newer compiler version in its GUI and queries the version of the compiler during initialisation, MRtrix3 doesn't actually see them. So here, I'm instead avoiding the "compiler:" directive altogether, and just setting CXX myself.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3