Bump postcss from 6.0.23 to 7.0.36

Bumps [postcss](https://github.com/postcss/postcss) from 6.0.23 to 7.0.36.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...7.0.36)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
...

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

package.json

@@ -40,7 +40,7 @@
     "gulp-uglify": "^3.0.2",
     "mocha": "^4.0.1",
     "open": "^6.4.0",
-    "postcss": "^6.0.19",
+    "postcss": "^7.0.36",
     "postcss-url": "^7.3.1",
     "sinon": "^4.1.3",
     "vinyl-buffer": "^1.0.1",