Bump gatsby-plugin-postcss from 2.1.7 to 2.1.8

Bumps [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss) from 2.1.7 to 2.1.8.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-postcss/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-postcss@2.1.8/packages/gatsby-plugin-postcss)

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

Author: dependabot-preview[bot]

package.json

@@ -10,7 +10,7 @@
     "gatsby-image": "^2.2.19",
     "gatsby-plugin-manifest": "^2.2.16",
     "gatsby-plugin-offline": "^3.0.6",
-    "gatsby-plugin-postcss": "^2.1.7",
+    "gatsby-plugin-postcss": "^2.1.8",
     "gatsby-plugin-react-helmet": "^3.1.7",
     "gatsby-plugin-sharp": "^2.2.22",
     "gatsby-source-filesystem": "^2.1.22",

yarn.lock

@@ -5191,10 +5191,10 @@ gatsby-plugin-page-creator@^2.1.17:
     lodash "^4.17.15"
     micromatch "^3.1.10"
 
-gatsby-plugin-postcss@^2.1.7:
-  version "2.1.7"
-  resolved "https://registry.yarnpkg.com/gatsby-plugin-postcss/-/gatsby-plugin-postcss-2.1.7.tgz#3ac4b0dc4db56f505114b7b722de9b1a4132d1e3"
-  integrity sha512-vZs1wuCYAmLOpOUdc8zaUWPcc5z0nY5LlPTQzv335C8UCgTiPdxxxVWJEe15I+axW/2hyec2wTXkQCTYssScLg==
+gatsby-plugin-postcss@^2.1.8:
+  version "2.1.8"
+  resolved "https://registry.yarnpkg.com/gatsby-plugin-postcss/-/gatsby-plugin-postcss-2.1.8.tgz#12d0a6396e2ac18edf827b242ce978eb85b29def"
+  integrity sha512-4ftxheD1O4sroglSd1W3Dity5V0XLwCq/z+hEJkzGEhR47w7O4cpuiYjuWg70D4f3l544m6RfBc5xnGEZCqkSg==
   dependencies:
     "@babel/runtime" "^7.6.0"
     postcss-loader "^3.0.0"