dependencies: gulp-minify-css deprecation

[gulp-minify-css](https://www.npmjs.com/package/gulp-minify-css) has been in deprecated in favor of [gulp-clean-css](https://github.com/scniro/gulp-clean-css). Please open an [issue](https://github.com/scniro/gulp-clean-css/issues) with the new repository if you do upgrade and experience any unexpected behavior. I look forward to maintaining this package moving forward.
