gulpfile2.js

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var atImport = require('postcss-import');
var autoprefixer = require('autoprefixer');
var cssnano = require('cssnano');

gulp.task('css', function () {
  var plugins = [
	atImport,
	autoprefixer,
	cssnano
  ];
  return gulp.src('./src/style.css')
    .pipe(postcss(plugins))
    .pipe(gulp.dest('./dest'));
});