Update to v3.5.1 and implement rails best practices

Updated this to use the rails way of doing things;  most notably moving the assets into the vendor folder.

* Moved `RAILS_ROOT/app/assets` to `RAILS_ROOT/vendor/assets`
* Replaced `select2.css.erb` with `select2.css.scss`
  * If we want to make this gem backwards compatible, we could maybe add `select2-scss.css.scss` instead (but I would recommend removing the CSS file ending in `erb`)
* Upgrade javascript and css to select2 v3.5.1
* Update `source_file.rb` so it converts the fetched assets to `scss` instead of `erb`