Bump gatsby-plugin-mdx from 1.0.40 to 1.0.42

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 1.0.40 to 1.0.42.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-mdx@1.0.42/packages/gatsby-plugin-mdx)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

yarn.lock

@@ -7338,9 +7338,9 @@ gatsby-plugin-google-analytics@^2.0.20:
     "@babel/runtime" "^7.6.0"
 
 gatsby-plugin-mdx@^1.0.5:
-  version "1.0.40"
-  resolved "https://registry.yarnpkg.com/gatsby-plugin-mdx/-/gatsby-plugin-mdx-1.0.40.tgz#be4e6f291c570ebff483d01f54f8d68b31a0b5b4"
-  integrity sha512-3pWNbqvywXsWpXTmUvK7FXfRhRnuKNgbxLB0gyLq1KbiyyDjuNuPmLGgOL4bVbXdKtSRedXtOGTiY5y6GM70Fg==
+  version "1.0.42"
+  resolved "https://registry.yarnpkg.com/gatsby-plugin-mdx/-/gatsby-plugin-mdx-1.0.42.tgz#2012d7406042050eedf78df61220ef4762548291"
+  integrity sha512-G+Wj8UWwIJR5qDAVQx40JgfBC5tNQ4D/dHrgs2SXAkYZV9EXTZBZ5CQtxwy95RlvC0R2VqjamXJmCbVkIZ6zBw==
   dependencies:
     "@babel/core" "^7.6.0"
     "@babel/generator" "^7.6.0"
@@ -7373,7 +7373,7 @@ gatsby-plugin-mdx@^1.0.5:
     static-site-generator-webpack-plugin "^3.4.2"
     style-to-object "^0.2.3"
     underscore.string "^3.3.5"
-    unified "^8.3.2"
+    unified "^8.4.0"
     unist-util-map "^1.0.5"
     unist-util-remove "^1.0.3"
     unist-util-visit "^1.4.1"
@@ -16392,7 +16392,7 @@ unicode-property-aliases-ecmascript@^1.0.4:
   resolved "https://registry.yarnpkg.com/unicode-property-aliases-ecmascript/-/unicode-property-aliases-ecmascript-1.0.5.tgz#a9cc6cc7ce63a0a3023fc99e341b94431d405a57"
   integrity sha512-L5RAqCfXqAwR3RriF8pM0lU0w4Ryf/GgzONwi6KnL1taJQa7x1TCxdJnILX59WIGOwR57IVxn7Nej0fz1Ny6fw==
 
-unified@8.3.2, unified@^8.3.2:
+unified@8.3.2:
   version "8.3.2"
   resolved "https://registry.yarnpkg.com/unified/-/unified-8.3.2.tgz#aed69d0e577d6ef27268431c63a10faef60e63ab"
   integrity sha512-NDtUAXcd4c+mKppCbsZHzmhkKEQuhveZNBrFYmNgMIMk2K9bc8hmG3mLEGVtRmSNodobwyMePAnvIGVWZfPdzQ==
@@ -16417,6 +16417,17 @@ unified@^7.0.0:
     vfile "^3.0.0"
     x-is-string "^0.1.0"
 
+unified@^8.4.0:
+  version "8.4.0"
+  resolved "https://registry.yarnpkg.com/unified/-/unified-8.4.0.tgz#5bb8a05c2a0b9dcc56152312ad8e4578a0d90af7"
+  integrity sha512-hQqeCrzqqS3vk8WbvbjYgaxe9WqmZF32Y3lz/kY5A8/5RdJbxoa4yOIAYpSEvqii9n2MTI2OL1+ByoVJYLhlUg==
+  dependencies:
+    bail "^1.0.0"
+    extend "^3.0.0"
+    is-plain-obj "^2.0.0"
+    trough "^1.0.0"
+    vfile "^4.0.0"
+
 union-value@^1.0.0:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/union-value/-/union-value-1.0.1.tgz#0b6fe7b835aecda61c6ea4d4f02c14221e109847"