build(deps): bump postcss from 7.0.27 to 7.0.28

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.27 to 7.0.28.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.27...7.0.28)

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

Author: dependabot-preview[bot]

packages/postcss-purgecss/package.json

@@ -26,7 +26,7 @@
     "url": "https://github.com/FullHuman/purgecss/issues"
   },
   "dependencies": {
-    "postcss": "7.0.27",
+    "postcss": "7.0.28",
     "purgecss": "^2.1.2"
   }
 }

packages/purgecss/package.json

@@ -41,7 +41,7 @@
   "dependencies": {
     "commander": "^5.0.0",
     "glob": "^7.0.0",
-    "postcss": "7.0.27",
+    "postcss": "7.0.28",
     "postcss-selector-parser": "^6.0.2"
   },
   "devDependencies": {