[Grunt] Cleaning your build folder with grunt-contrib-clean

npm install grunt-contrib-clean --save-dev

clean: {
            build: ‘build‘  //clean the build directory
        }

grunt clean:build

/**
 * Created by Answer1215 on 11/16/2014.
 */
module.exports = function(grunt) {
    grunt.initConfig({
        stylus:{
            compile:{
                options: {
                    compress: false
                },
                files: {
                    "app/css/app.css": "styl/app.styl"
                }
            }
        },
        watch:{
            stylus:{
                files: [‘styl/**/*.styl‘],
                tasks: [‘stylus:compile‘]
            },
            css:{
                options: {livereload: true},
                files: [‘app/css/**.css‘]
            },
            html:{
                options: {livereload: true},
                files: [‘**.html‘]
            },
            script: {
                options: {livereload: true},
                files: [‘app/js/**.js‘]
            }
        },
        concat:{
            options: {
                separator: ‘;‘
            },
            js:{
                src: [‘bower_components/angular/angular.min.js‘, ‘app/js/app.js‘, ‘app/js/**.js‘],
                dest: "build/app.js"
            }
        },
        uglify: {
            js: {
                src: ["build/app.js"],
                dest: "build/app.min.js"
            }
        },
        clean: {
            build: ‘build‘  //clean the build directory
        }
    });

    grunt.registerTask(‘build‘);

    grunt.loadNpmTasks(‘grunt-contrib-watch‘);
    grunt.loadNpmTasks(‘grunt-contrib-stylus‘);
    grunt.loadNpmTasks(‘grunt-contrib-concat‘);
    grunt.loadNpmTasks(‘grunt-contrib-uglify‘);
    grunt.loadNpmTasks(‘grunt-contrib-clean‘);
}