silence psutil warning

I get the following warning from psutil after importing distributed:
```
>>> import distributed
/nas1/Data_arco/prgr/Anaconda2/lib/python2.7/site-packages/psutil/_pslinux.py:338: RuntimeWarning: shared memory stats couldn't be determined and was set to 0
warnings.warn(msg, RuntimeWarning)
```

In dask, this warning was silenced: https://github.com/dask/dask/issues/1588

Should the same be done here?