build: bump postcss-loader from 2.1.1 to 2.1.2 (#72)

Bumps [postcss-loader](https://github.com/postcss/postcss-loader) from 2.1.1 to 2.1.2.
- [Release notes](https://github.com/postcss/postcss-loader/releases)
- [Changelog](https://github.com/postcss/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/postcss-loader@v2.1.1...v2.1.2)

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