Bump postcss from 7.0.17 to 7.0.18

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.17 to 7.0.18.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.17...7.0.18)

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

Author: dependabot-preview[bot]

yarn.lock

@@ -3889,9 +3889,9 @@ postcss@^6.0.9:
     supports-color "^5.4.0"
 
 postcss@^7.0.11, postcss@^7.0.14, postcss@^7.0.17:
-  version "7.0.17"
-  resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.17.tgz#4da1bdff5322d4a0acaab4d87f3e782436bad31f"
-  integrity sha512-546ZowA+KZ3OasvQZHsbuEpysvwTZNGJv9EfyCQdsIDltPSWHAeTQ5fQy/Npi2ZDtLI3zs7Ps/p6wThErhm9fQ==
+  version "7.0.18"
+  resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.18.tgz#4b9cda95ae6c069c67a4d933029eddd4838ac233"
+  integrity sha512-/7g1QXXgegpF+9GJj4iN7ChGF40sYuGYJ8WZu8DZWnmhQ/G36hfdk3q9LBJmoK+lZ+yzZ5KYpOoxq7LF1BxE8g==
   dependencies:
     chalk "^2.4.2"
     source-map "^0.6.1"