Gulpfile.js for Jekyll site with critical CSS

Gulpfile.js

// Require Gulp
var gulp = require('gulp');
var sass = require('gulp-sass');
var sourcemaps = require('gulp-sourcemaps');
var prefix = require('gulp-autoprefixer');
var watch = require('gulp-watch');
var critical = require('critical');

// Gulp Sass Task 
gulp.task('sass', function() {
  gulp.src('_sass/**/*.scss')    
    .pipe(sourcemaps.init()) // Initializes sourcemaps
    .pipe(sass({
      errLogToConsole: true,
      outputStyle: "compressed"
      }))
    .pipe(prefix('last 2 versions', '> 1%', 'ie 8'))
    .pipe(sourcemaps.write('./')) // Writes sourcemaps into the CSS file
    .pipe(gulp.dest('css'));
})

gulp.task('critical', function () {
    critical.generate({
        base: './',
        src: '_site/index.html',
        css: '_site/css/screen.css',
        dest: '_includes/critical.css',
        width: 320,
        height: 480,
        minify: true
    });
});

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

gulp.task('default', ['sass', 'watch']);