Fix VIIRS HNCC composite passing xarray objects to dask

[djhoese]

See dask/dask#4532 for details. This fixes the poor usage of dask by passing xarray objects to it.

I'm also using this PR to remove the bdist_wheel creation in travis CI given the recent issues discovered with versioneer and wheel creation (see python-versioneer/python-versioneer#170).

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[codecov]

Codecov Report

Merging #633 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
- Coverage   78.29%   78.28%   -0.01%     
==========================================
  Files         136      136              
  Lines       19725    19723       -2     
==========================================
- Hits        15443    15441       -2     
  Misses       4282     4282

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/test_viirs.py	99.28% <ø> (-0.01%)	⬇️
satpy/composites/viirs.py	83.71% <100%> (-0.04%)	⬇️

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[coveralls]

Coverage decreased (-0.002%) to 78.289% when pulling df5c786 on djhoese:bugfix-hncc-dnb into ef5dc2c on pytroll:master.