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.