Use travis tag for master
Use travis tag for master

Only indicate travis status for master branch. Fixes #1771.

dask/dask,mrocklin/dask,dask/dask,mrocklin/dask,blaze/dask,jcrist/dask,cpcloud/dask,ContinuumIO/dask,jakirkham/dask,mraspaud/dask,blaze/dask,ContinuumIO/dask,mraspaud/dask,jakirkham/dask,gameduell/dask,jcrist/dask,chrisbarber/dask