Update to fix deprecation warning from dask regarding atop

[djhoese]

A user reported an error on slack that also showed a deprecation warning in dask. This may affect both satpy and pyresample:

/home/raghav/anaconda2/lib/python2.7/site-packages/dask/array/blockwise.py:204: UserWarning: The da.atop function has moved to da.blockwise
  warnings.warn("The da.atop function has moved to da.blockwise")

We should switch to using blockwise instead of atop when possible.

@mraspaud Maybe we should make all UserWarning/DeprecationWarnings raise errors on travis.

[mraspaud]

Raising on travis sounds good.

[djhoese]

I think we updated these usages a long time ago. And we can raise all warnings on travis (now github actions) as we sometimes deliberately test deprecated interfaces of other libraries. We do have an "unstable" environment which should catch things when they are completely removed from upstream packages.