Update Gutenberg ref in package.json

[ellatrix]

Trac ticket: https://core.trac.wordpress.org/ticket/64595

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[desrosj]

Do we have a good way to track which changes (features, bugs, etc.) are being included in each ref update? Apologies if I missed it, but are we bumping to refs representing specific changes, such as releases or certain milestone completions?

Without the SVN commit messages noting what is included in each ref bump, there will inevitably be situations where tracking the introduction of certain changes is near impossible.