only run travis CI on master branch changes (#3319)
only run travis CI on master branch changes (#3319)

travis will run on PR automatically but we don't need 2 runs (pr branch) as well.
zooniverse/Panoptes,camallen/Panoptes,camallen/Panoptes,camallen/Panoptes,zooniverse/Panoptes,zooniverse/Panoptes,zooniverse/Panoptes,camallen/Panoptes