Update dependency load-grunt-tasks to v5

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
load-grunt-tasks	4.0.0 -> 5.1.0

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Release Notes

sindresorhus/load-grunt-tasks

### [`v5.1.0`](https://togithub.com/sindresorhus/load-grunt-tasks/releases/v5.1.0) [Compare Source](https://togithub.com/sindresorhus/load-grunt-tasks/compare/v5.0.0...v5.1.0) - Print a useful error if package.json is missing ([#​61](https://togithub.com/sindresorhus/load-grunt-tasks/issues/61)) [`6234bc9`](https://togithub.com/sindresorhus/load-grunt-tasks/commit/6234bc9) ### [`v5.0.0`](https://togithub.com/sindresorhus/load-grunt-tasks/releases/v5.0.0) [Compare Source](https://togithub.com/sindresorhus/load-grunt-tasks/compare/v4.0.0...v5.0.0) Breaking: - Require Node.js 8 [`a928bc7`](https://togithub.com/sindresorhus/load-grunt-tasks/commit/a928bc7) Enhancements: - Upgrade dependencies [`a928bc7`](https://togithub.com/sindresorhus/load-grunt-tasks/commit/a928bc7)

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