.travis.yml: Unpin `conda-build` version.
.travis.yml: Unpin `conda-build` version.

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