Try conda 4.7 again.

[ueshin]

Resolves #508.

[codecov-io]

Codecov Report

Merging #517 into master will decrease coverage by 2.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #517      +/-   ##
==========================================
- Coverage   93.64%   91.38%   -2.27%     
==========================================
  Files          31       31              
  Lines        4188     4188              
==========================================
- Hits         3922     3827      -95     
- Misses        266      361      +95

Impacted Files	Coverage Δ
databricks/koalas/usage_logging/__init__.py	23.58% <0%> (-73.59%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	76.31% <0%> (-7.9%)	⬇️
databricks/conftest.py	90.9% <0%> (-6.07%)	⬇️

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[ueshin]

This is the similar problem conda/conda#8843.
conda/conda#8844 might be a problem, too.
We should skip conda 4.7.5. Let's see what happens in 4.7.6 or upper.