Add support for lists with `<optgroup>`'s

When translating a list with `<optgroup>`'s select2 will use the
following structue:
````
div.select2-drop
  ul.select2-results
    li.select2-result.select2-result-unselectable
      div
      ul.select2-result-stub
        li.select2-result.select2-result-selectable
        li.select2-result.select2-result-selectable
        ...
    li.select2-result.select2-result-unselectable
    ...
````
The old `"#{drop_container} li"` selector would lead to
`Capybara::Ambiguous` error complaining that:

Ambiguous match, found 2 elements matching css ".select2-drop li" with
text "..." 

because of the additional `<li>` at the optgroup level.

This change adds a selector for `"select2-result-selectable"` to
ignore such headers.