Reset language select after adding a language

It worked differently after upgrading to jQuery 3. According to the
jQuery upgrade guide:

> It is almost always a mistake to use .removeAttr("checked") on a DOM
> element. The only time it might be useful is if the DOM is later going
> to be serialized back to an HTML string. In all other cases,
> .prop("checked", false) should be used instead.

app/assets/javascripts/globalize.js

@@ -7,7 +7,7 @@
     display_locale: function(locale) {
       App.Globalize.enable_locale(locale);
       App.Globalize.add_language(locale);
-      $(".js-add-language option:selected").removeAttr("selected");
+      $(".js-add-language option:selected").prop("selected", false);
     },
     display_translations: function(locale) {
       $(".js-select-language option[value=" + locale + "]").prop("selected", true);

spec/system/admin/translatable_spec.rb

@@ -493,6 +493,7 @@ describe "Admin edit translatable records" do
       select "Français", from: :add_language
 
       expect_to_have_language_selected "Français"
+      expect(page).to have_select :add_language, selected: "Add language"
       expect(page).to have_field "contents_content_#{content.key}values_value_fr"
     end