build(deps): bump postcss from 7.0.25 to 7.0.26 (#570)

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.25 to 7.0.26.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.25...7.0.26)

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

yarn.lock

@@ -4679,9 +4679,9 @@ postcss-selector-parser@^6.0.2:
     uniq "^1.0.1"
 
 postcss@^7.0.21:
-  version "7.0.25"
-  resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.25.tgz#dd2a2a753d50b13bed7a2009b4a18ac14d9db21e"
-  integrity sha512-NXXVvWq9icrm/TgQC0O6YVFi4StfJz46M1iNd/h6B26Nvh/HKI+q4YZtFN/EjcInZliEscO/WL10BXnc1E5nwg==
+  version "7.0.26"
+  resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.26.tgz#5ed615cfcab35ba9bbb82414a4fa88ea10429587"
+  integrity sha512-IY4oRjpXWYshuTDFxMVkJDtWIk2LhsTlu8bZnbEJA4+bYT16Lvpo8Qv6EvDumhYRgzjZl489pmsY3qVgJQ08nA==
   dependencies:
     chalk "^2.4.2"
     source-map "^0.6.1"