Remove dask warning

[jbusecke]

The issue was fixed in xarray 0.15

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   46.41%   46.45%   +0.04%     
==========================================
  Files          13       13              
  Lines        1312     1313       +1     
  Branches      296      296              
==========================================
+ Hits          609      610       +1     
  Misses        654      654              
  Partials       49       49

Impacted Files	Coverage Δ
xarrayutils/utils.py	40.14% <ø> (-0.22%)	⬇️
xarrayutils/_version.py	36.91% <0%> (+0.45%)	⬆️

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