Update to v3.5.1 and implement rails best practices

Updated to use rails best practices:

* Moved `RAILS_ROOT/app/assets` to `RAILS_ROOT/vendor/assets`
* Replaced `select2.css.erb` with `select2.css`
* Update `source_file.rb` so it converts the fetched css to use rails paths instead of erb
* Upgrade javascript and css to select2 v3.5.1