Schema version added for entityset serialization

[alexjwang]

fixes #556

[codecov]

Codecov Report

Merging #586 into master will increase coverage by 0.01%.
The diff coverage is 98.36%.

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
+ Coverage   96.44%   96.46%   +0.01%     
==========================================
  Files         117      117              
  Lines        9534     9578      +44     
==========================================
+ Hits         9195     9239      +44     
  Misses        339      339

Impacted Files	Coverage Δ
featuretools/entityset/serialize.py	100% <100%> (ø)	⬆️
featuretools/tests/entityset_tests/test_es.py	100% <100%> (ø)	⬆️
featuretools/feature_base/features_deserializer.py	100% <100%> (+1.75%)	⬆️
featuretools/entityset/deserialize.py	100% <100%> (ø)	⬆️
featuretools/utils/gen_utils.py	91.54% <95.45%> (+1.75%)	⬆️

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