fix(deps): update dependency gatsby-plugin-sharp to ^2.3.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby-plugin-sharp	dependencies	patch	^2.3.0 -> ^2.3.1

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Release Notes

gatsbyjs/gatsby

### [`v2.3.1`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.3.0...gatsby@2.3.1) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/1e6954a0e8e8a661a58bc3421a3d17d8e38e766c...314526e2bc28715d83ffde0e6c172b483db1184c)

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