Add 4.1.0 to supported Chainer version list

[laurenyu]

noticed this when I was looking at the README. figured I might as well put out a quick PR.

Merge Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[codecov-io]

Codecov Report

Merging #378 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #378   +/-   ##
=======================================
  Coverage   93.07%   93.07%           
=======================================
  Files          51       51           
  Lines        3568     3568           
=======================================
  Hits         3321     3321           
  Misses        247      247

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