Bump @babel/preset-env from 7.5.5 to 7.6.0 in /slides

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.5.5 to 7.6.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](babel/babel@v7.5.5...v7.6.0)

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