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 `<li>` at the optgroup level.

This change adds a selector for `"select2-result-selectable"` to
ignore such headers.
Merge pull request #28 from maissimples/optgroup_support

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