run bower install

Gruntfile.js

module.exports = function(grunt) {
    require('load-grunt-tasks')(grunt);
    // Project configuration.
    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),
        shell: { // Task
            bower_update: { // Target
                options: { // Options
                    stderr: false
                },
                command: 'bower update'
            }
        },
        watch: {
            scripts: {
                files: ['bower.json'],
                tasks: ['shell:bower_update'],
                options: {
                    spawn: false,
                }
            }
        }

    });
};