Pin scikit-learn to version supporting Py2

[brianmartin]

See tensorflow/data-validation#64
This commit may be reverted if that PR is merged.

[codecov]

Codecov Report

Merging #195 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #195   +/-   ##
=======================================
  Coverage   89.11%   89.11%           
=======================================
  Files          12       12           
  Lines         744      744           
=======================================
  Hits          663      663           
  Misses         81       81

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