Update manifest when copying assets

If the [pull request #104][tinymce-rails-pulls-104] in [tinymce-rails][] is getting merged, this pull request should prevent assets from getting deleted by the Capistrano `deploy:assets:clean_expired` task. (The same problem was once reported and fixed in spohlenz/tinymce-rails#83.)

The changes in this pull request are basically copied from the [lib/tasks/tinymce-assets.rake][] file in [tinymce-rails][].

Please have a look. Thanks! :smile: 

[tinymce-rails-pulls-104]: https://github.com/spohlenz/tinymce-rails/pull/104
[tinymce-rails]: https://github.com/spohlenz/tinymce-rails
[lib/tasks/tinymce-assets.rake]: https://github.com/spohlenz/tinymce-rails/blob/master/lib/tasks/tinymce-assets.rake