Update gatsby monorepo

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
babel-preset-gatsby	devDependencies	patch	0.2.22 -> 0.2.23
gatsby	dependencies	patch	2.18.3 -> 2.18.4
gatsby-image	dependencies	patch	2.2.33 -> 2.2.34
gatsby-plugin-emotion	dependencies	patch	4.1.15 -> 4.1.16
gatsby-plugin-less	dependencies	patch	3.0.14 -> 3.0.15
gatsby-plugin-manifest	dependencies	patch	2.2.29 -> 2.2.30
gatsby-plugin-offline	dependencies	patch	3.0.23 -> 3.0.24
gatsby-plugin-react-helmet	dependencies	patch	3.1.15 -> 3.1.16
gatsby-plugin-sharp	dependencies	patch	2.3.3 -> 2.3.4
gatsby-source-filesystem	dependencies	patch	2.1.38 -> 2.1.39
gatsby-transformer-remark	dependencies	patch	2.6.37 -> 2.6.38
gatsby-transformer-sharp	dependencies	patch	2.3.5 -> 2.3.6

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Release Notes

gatsbyjs/gatsby

### [`v0.2.23`](https://togithub.com/gatsbyjs/gatsby/compare/e6a30aaadc6949d125eaf4d16c8fb36ebafad10d...168bb344d6a5e7f71aa4f3812fd3fb22ae0a7416) [Compare Source](https://togithub.com/gatsbyjs/gatsby/compare/e6a30aaadc6949d125eaf4d16c8fb36ebafad10d...168bb344d6a5e7f71aa4f3812fd3fb22ae0a7416)

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[codecov[bot]]

Codecov Report

Merging #365 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #365   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          60     60           
  Branches       10     10           
=====================================
  Hits           60     60

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