Error starting gulp

var gulp = require('gulp'), sass = require('gulp-sass'), browserSync = require('browser-sync'), concat = require('gulp-concat'), uglify = require('gulp-uglifyjs'), cssnano = require('gulp-cssnano'), rename = require('gulp-rename'), del = require('del'), autoprefixer = require('gulp-autoprefixer'), csslint = require('gulp-csslint'), shorthand = require('gulp-shorthand');; gulp.task('sass', function(){ return gulp.src('app/sass/**/*.+(scss|sass)') .pipe(sass()) .pipe(autoprefixer(['last 15 versions', '> 1%', 'ie 8', 'ie 7'], { cascade: true })) .pipe(csslint()) .pipe(shorthand()) .pipe(gulp.dest('app/css')) .pipe(browserSync.reload({stream: true})) }); gulp.task('scripts', function(){ return gulp.src([ 'app/libs/jquery/dist/jquery.min.js' ]) .pipe(concat('libs.min.js')) .pipe(uglify()) .pipe(gulp.dest('app/js')); }); gulp.task('css-libs', ['sass'], function(){ return gulp.src('app/css/libs.css') .pipe(cssnano()) .pipe(rename({suffix: '_min'})) .pipe(gulp.dest('app/css')); }); gulp.task('browser-sync', function(){ browserSync({ server: { baseDir: 'app' }, notify: false }); }); gulp.task('clean', function(){ return del.sync('dist'); }); gulp.task('watch', ['browser-sync', 'css-libs', 'scripts'], function(){ gulp.watch('app/sass/**/*.+(scss|sass)', ['sass']); gulp.watch('app/*.html', browserSync.reload); gulp.watch('app/js/**/*.js', browserSync.reload); }); gulp.task('build', ['clean', 'sass', 'scripts'], function(){ var buildCss = gulp.src('app/css/**/*') .pipe(gulp.dest('dist/css')); var buildFonts = gulp.src('app/fonts/**/*') .pipe(gulp.dest('dist/fonts')); var buildJs = gulp.src('app/js/**/*') .pipe(gulp.dest('dist/js')); var buildHtml = gulp.src('app/*.html') .pipe(gulp.dest('dist')); var buildPhp = gulp.src('app/*.php') .pipe(gulp.dest('dist')); var buildImg = gulp.src('app/img/**/*') .pipe(gulp.dest('dist/img')); });