Update gatsby monorepo

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gatsby	dependencies	patch	2.18.5 -> 2.18.6
gatsby-plugin-manifest	dependencies	patch	2.2.30 -> 2.2.31
gatsby-plugin-offline	dependencies	patch	3.0.25 -> 3.0.26
gatsby-plugin-sharp	dependencies	patch	2.3.4 -> 2.3.5
gatsby-source-filesystem	dependencies	patch	2.1.39 -> 2.1.40
gatsby-transformer-remark	dependencies	patch	2.6.38 -> 2.6.39

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Release Notes

gatsbyjs/gatsby

### [`v2.18.6`](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.5...gatsby@2.18.6) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/gatsby@2.18.5...gatsby@2.18.6)

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[codecov[bot]]

Codecov Report

Merging #371 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #371   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          60     60           
  Branches       10     10           
=====================================
  Hits           60     60

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