Update download-readmes.js -> Babylon

[QC-L]

Babylon has been moved into the main Babel mono-repo.
Change download uri to /babel/babel/master/packages/babylon/README.md.

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[yavorsky]

Thanks

[QC-L]

@yavorsky Not at all.

[yavorsky]

Hmm.. I just remembered why we haven't done that before...
Actually, we haven't released 7 yet, so the website doesn't refer to any 7 stuff.

Look at the babel-preset-env docs uri: https://github.com/QC-L/babeljs.io/blob/b3e134dc6fb068ae6be04236afe1d7a1b9718f9d/scripts/download-readmes.js#L44

So we need to be consistent and update preset-env also since we've moved it to the monorepo like Babylon, or use Babylon's 6.x branch because preset-env refers to 1.x.
I'd prefer the 2 option unless Babel 7 released.

Sorry for the initial confusion.

[QC-L]

@yavorsky I see. Thank you. Look forward to the release of Babel 7.

[yavorsky]

@QC-L You can update this PR with a branch name change to 6.x. 🙂

[QC-L]

Nothing, no hurry. Wait for you to update.

[hzoo]

Yeah with #1485 - we should have versioned docs so it won't be as confusing

[hzoo]

Can close now since we have the new docs