.travis.yml: Add GNU compilers in `apt` section.
.travis.yml: Add GNU compilers in `apt` section.

DudLab/rank_filter,jakirkham/rank_filter,DudLab/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter