Use a fixed version for GCC on Travis / OS X
Use a fixed version for GCC on Travis / OS X

chemfiles/chemfiles,Luthaf/Chemharp,chemfiles/chemfiles,lscalfi/chemfiles,Luthaf/Chemharp,lscalfi/chemfiles,lscalfi/chemfiles,lscalfi/chemfiles,chemfiles/chemfiles,Luthaf/Chemharp,chemfiles/chemfiles