Ошибка при запуске 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'));
});