chore(deps): update gatsby monorepo

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
babel-preset-gatsby	devDependencies	patch	0.5.14 -> 0.5.15
gatsby (changelog)	dependencies	patch	2.24.91 -> 2.24.92
gatsby-plugin-sharp	dependencies	patch	2.6.43 -> 2.6.44
gatsby-source-contentful	dependencies	patch	3.0.6 -> 3.0.7
gatsby-transformer-sharp	dependencies	patch	2.5.20 -> 2.5.21

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Release Notes

gatsbyjs/gatsby

### [`v0.5.15`](https://togithub.com/gatsbyjs/gatsby/compare/e2605d79f3815c31c1c3458e48d15dc23256f4bd...5fbfaa81260fabc88aa920b04f3d51ea568323d0) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/e2605d79f3815c31c1c3458e48d15dc23256f4bd...5fbfaa81260fabc88aa920b04f3d51ea568323d0)

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