Drop old `numpy` version stuff from Travis tests
Drop old `numpy` version stuff from Travis tests

No longer need to fix the `numpy` version as we let it float freely as
long as it is at least 1.11.

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