Only install pytest-runner if needed (#2692)
Only install pytest-runner if needed (#2692)

Only add pytest-runner to `setup_requires` if doing a test invocation.
mrocklin/dask,blaze/dask,mrocklin/dask,dask/dask,blaze/dask,jakirkham/dask,dask/dask,jcrist/dask,ContinuumIO/dask,jcrist/dask,jakirkham/dask,ContinuumIO/dask