Update dependency grunt-gh-pages to v3

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
grunt-gh-pages	devDependencies	major	2.1.0 -> 3.1.0

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Release Notes

tschaub/grunt-gh-pages

### [`v3.1.0`](https://togithub.com/tschaub/grunt-gh-pages/blob/master/changelog.md#​310) [Compare Source](https://togithub.com/tschaub/grunt-gh-pages/compare/v3.0.0...v3.1.0) Downgrade to `q-io@1.13.6` for a security fix (see [#​73][#​73]) ### [`v3.0.0`](https://togithub.com/tschaub/grunt-gh-pages/blob/master/changelog.md#​300) [Compare Source](https://togithub.com/tschaub/grunt-gh-pages/compare/v2.1.0...v3.0.0) Updated dependencies and stopped testing on Node 4 (see [#​72][#​72])

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