Always install gcc-8 on travis, but exclude g++-8 for the C compilation tests.
Always install gcc-8 on travis, but exclude g++-8 for the C compilation tests.

scoder/cython,da-woods/cython,cython/cython,scoder/cython,scoder/cython,da-woods/cython,cython/cython,da-woods/cython,scoder/cython,cython/cython,da-woods/cython,cython/cython