fix(deps): Fixes Grunt peer dependency configuration

[timnolte]

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• Does your code follow the coding style configured in the .editoconfig?
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Changes proposed in this Pull Request:

• Updates the Grunt peer dependency for compatibility with all 1.x versions.
• Fixes the README license badge.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
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Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

• Updates the Grunt peer dependency for compatibility with all 1.x versions.