docs: replace es2015 with env

[kentcdodds]

Because babel-preset-es2015 is deprecated and babel-preset-env is a drop-in replacement.

Summary

Replace es2015 with env because babel-preset-es2015 is deprecated and babel-preset-env is a drop-in replacement.

Test plan

N/A

[codecov-io]

Codecov Report

Merging #5385 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5385   +/-   ##
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  Coverage   61.33%   61.33%           
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  Files         205      205           
  Lines        6924     6924           
  Branches        4        3    -1     
=======================================
  Hits         4247     4247           
  Misses       2676     2676           
  Partials        1        1

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[thymikee]

Awesome! Would you mind also replacing it in README.md? https://github.com/facebook/jest#using-babel

[kentcdodds]

Sure thing!

[kentcdodds]

Done 👍

[thymikee]

❤️

[kentcdodds]

Note there are still some integration tests using that preset. Perhaps if I think of it later I'll open another PR for it 👍

[thymikee]

Yup, noticed that, but it's unrelated to PR topic. Much appreciated!

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