Update requirements.txt

[mathavraj]

sklearn is a misnomer for the actual scikit-learn library #35
@mikkokotila @lahdjirayhan

You want to make a PR to Wrangle

Thanks so much :) First, please take a moment to carefully check through
the below items:

Sanity

• I'm aware of the implications of the proposed changes
• Code is PEP8
• I'm making the PR to master

Docs

• Docs are updated
• Docs version is correct (index.html and _coverpage.md)

Tests

• Changes have gone through actual use testing
• All local tests have passed (run ./test.sh in /talos)
• Tests have been updated to reflect the changes

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