fix(deps): update gatsby

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby	dependencies	minor	^2.17.15 -> ^2.18.2
gatsby-plugin-google-analytics	dependencies	patch	^2.1.27 -> ^2.1.28
gatsby-plugin-manifest	dependencies	patch	^2.2.28 -> ^2.2.29
gatsby-plugin-offline	dependencies	patch	^3.0.21 -> ^3.0.22
gatsby-plugin-sharp	dependencies	patch	^2.3.1 -> ^2.3.2

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Release Notes

gatsbyjs/gatsby

### [`v2.18.2`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.1...gatsby@2.18.2) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.1...gatsby@2.18.2) ### [`v2.18.1`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.0...gatsby@2.18.1) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.0...gatsby@2.18.1) ### [`v2.18.0`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.17.17...gatsby@2.18.0) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.17.17...gatsby@2.18.0) ### [`v2.17.17`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.17.16...gatsby@2.17.17) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.17.16...gatsby@2.17.17) ### [`v2.17.16`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.17.15...gatsby@2.17.16) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.17.15...gatsby@2.17.16)

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