增加 underscore 依赖，解决编译出错

如果没有这行，Grunt 将提示编译出错：

    $ grunt 
    Running "clean:dist" (clean) task
    Cleaning build/...OK
    Cleaning tmp/...OK

    Running "bower:install" (bower) task
    >> Cleaned target dir /Users/kuangxuqing/Documents/Projects/chaplin-with-browserify/tmp/vendor
    >> Installed bower packages
    >> Copied packages to /Users/kuangxuqing/Documents/Projects/chaplin-with-browserify/tmp/vendor

    Running "copy:assets" (copy) task
    Copied 2 files

    Running "stylus:dist" (stylus) task
    File tmp/css/app.css created.

    Running "browserify:app" (browserify) task
    >> Error: Cannot find module 'underscore'
    Warning: Error running grunt-browserify. Use --force to continue.

    Aborted due to warnings.