Allow 0.20 version of cargo generate.

[abizjak]

Purpose

Fixes #415

The update is safe since 0.20 is a maintenace release https://github.com/cargo-generate/cargo-generate/releases/tag/v0.20.0

Checklist

• [ ] My code follows the style of this project.
• [ ] The code compiles without warnings.
• [ ] I have performed a self-review of the changes.
• [ ] I have documented my code, in particular the intent of the hard-to-understand areas.
• [ ] (If necessary) I have updated the CHANGELOG.