Gulp PostCSS Configuration

gulpfile.js

var gulp = require('gulp');
var postcssImport = require('postcss-import');
var autoprefixer = require('autoprefixer');
var customProperties = require("postcss-custom-properties");
var calc = require("postcss-calc")
var processors = [ postcssImport(), customProperties(), calc() , autoprefixer({browsers: ['> 1%','last 2 versions']}) ];
var postcss = require('gulp-postcss');
var rename = require("gulp-rename");
var minimist = require('minimist');
var options;


gulp.task('css', function () {
    return gulp.src(['./src/style/style.css'])
    .pipe( postcss(processors) )
    .pipe( rename("core.css") )
    .pipe( gulp.dest('assets/css/') );
});

gulp.task('css:watch', function () {
    gulp.watch(['./src/style/*.css'], ['css']);
});

package.json

{
  "name": "gulpPostcss",
  "version": "1.0.0",
  "description": "gulpPostcss",
  "private": true,
  "license": "UNLICENSED",
  "devDependencies": {
    "autoprefixer": "6.0.1",
    "gulp": "3.9.0",
    "gulp-postcss": "6.0.1",
    "postcss-calc": "5.0.0",
    "postcss-custom-properties": "5.0.0",
    "postcss-import": "7.1.3"
  }
}