[MRG] Update sklearn minimum version

[fferegrino]

Reference Issue

Fixes #839

What does this implement/fix? Explain your changes.

The minimum version for scikit-learn was outdated; the change was introduced in this commit f40e654 but the README.rst was never updated.

Any other comments?

[codecov]

Codecov Report

Merging #840 (3bb2771) into master (3748569) will decrease coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #840      +/-   ##
==========================================
- Coverage   98.69%   98.26%   -0.43%     
==========================================
  Files          93       93              
  Lines        6063     6063              
  Branches      508      508              
==========================================
- Hits         5984     5958      -26     
- Misses         78      104      +26     
  Partials        1        1              

Impacted Files	Coverage Δ
imblearn/datasets/tests/test_zenodo.py	56.75% <0.00%> (-32.44%)	⬇️
imblearn/datasets/_zenodo.py	74.19% <0.00%> (-22.59%)	⬇️

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[hayesall]

The codecov error looks unrelated, might have been a temporary network error on one of the CI jobs when accessing zenodo.

Looks good to me!