build(deps): bump postcss from 6.0.23 to 7.0.39 in /demo/vue-cli4

Bumps [postcss](https://github.com/postcss/postcss) from 6.0.23 to 7.0.39.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/7.0.39/CHANGELOG.md)
- [Commits](postcss/postcss@6.0.23...7.0.39)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>