Update postcss requirement from 5.0.12 to 7.0.6
Update postcss requirement from 5.0.12 to 7.0.6

Updates the requirements on [postcss](https://github.com/postcss/postcss) to permit the latest version.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/commits/7.0.6)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
hexojs/hexo-autoprefixer