.travis.yml: Install VIGRA conditionally for testing purposes.
.travis.yml: Install VIGRA conditionally for testing purposes.

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