.travis.yml: Fix indentation to 2 spaces.
.travis.yml: Fix indentation to 2 spaces.

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