gulp setup.

gulpfile.js

var
    //modules
    gulp = require('gulp'),
    // css modules
    sass = require('gulp-sass'),
    postcss = require('gulp-postcss'),
    assets = require('postcss-assets'),
    autoprefixer = require('autoprefixer'),
    mqpacker = require('css-mqpacker'),
    cssnano = require('cssnano');

gulp.task('sass', function () {
    var postCssOpts = [
        autoprefixer({ browsers: ['last 2 versions', '> 2%', 'ie > 7'] }),
        mqpacker
    ];
    return gulp.src('scss/**/*')
      .pipe(sass({
          outputStyle: 'expanded',
          precisions: 3,
          errLogToConsole: true
      })) // Converts Sass to CSS with gulp-sass
      .pipe(postcss(postCssOpts))
      .pipe(gulp.dest('../css'))
});

gulp.task('watch', function () {
    gulp.watch('scss/**/*', ['sass']);
});

package.json

{
  "name": "",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "directories": {
    "test": "tests"
  },
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "autoprefixer": "^6.7.7",
    "css-mqpacker": "^5.0.1",
    "cssnano": "^3.10.0",
    "gulp": "^3.9.1",
    "gulp-postcss": "^6.4.0",
    "gulp-sass": "^3.1.0",
    "postcss-assets": "^4.1.0",
    "srcset": "^1.0.0"
  }
}