Sass + Jekyll + BrowserSync

gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');
var uncss = require('gulp-uncss');
var prefix = require('gulp-autoprefixer');
var browserSync = require('browser-sync');

gulp.task('sass', function () {
    gulp.src('_sass/index.scss')
    .pipe(sass({outputStyle: 'compressed'}, {errLogToConsole: true}))
    .pipe(prefix("last 2 versions", "> 1%", "ie 8", "Android 2", "Firefox ESR"))
    .pipe(gulp.dest('css'))
    .pipe(browserSync.reload({stream:true}));
});

gulp.task('jekyll', function () {
	require('child_process').spawn('jekyll', ['build'], {stdio: 'inherit'});
});

gulp.task('browser-sync', function() {
    browserSync.init(null, {
      proxy: "localhost:4000"
    });
});

gulp.task('default', ['sass', 'browser-sync'], function () {
	 require('child_process').spawn('jekyll', ['serve'], {stdio: 'inherit'});
	gulp.watch("_sass/*.scss", ['sass']);
});