Fix DatetimeIndex variable conversion

[gsheni]

• Fixes featuretools doesn't like datetime64[ns, UTC] #300

[codecov]

Codecov Report

Merging #342 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
- Coverage   95.35%   95.34%   -0.01%     
==========================================
  Files          74       74              
  Lines        7791     7799       +8     
==========================================
+ Hits         7429     7436       +7     
- Misses        362      363       +1

Impacted Files	Coverage Δ
featuretools/entityset/entity.py	95.65% <100%> (ø)	⬆️
featuretools/tests/entityset_tests/test_es.py	99.23% <100%> (+0.01%)	⬆️
featuretools/utils/gen_utils.py	85.71% <0%> (-2.39%)	⬇️

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[kmax12]

Looks good to merge