Rename folder vendor to app

After @wongyouth commented in [this issue](https://github.com/bai/facebox-rails/issues/8) I followed [this link](https://github.com/rails/rails/pull/7968) and figured it out that Rails 4 is not looking in ```vendor``` nor ```lib``` folders for assets compilation, instead Rails look into the ```app``` folder.

So I forked facebox-rails and renamed the folder ```vendor``` to ```app``` and [tested with a Rails 4 app](http://whispering-taiga-7557.herokuapp.com/) hosted by Heroku and it worked like a charm :heart: 

I'm happy to contribute.