Let Globalize use I18n locale

This is a mistake I made in commit f2ef27d3. Back then I thought we
needed to keep Globalize.locale and I18n.locale in sync, but the truth
is it automatically happens when setting Globalize.locale to nil.

So now we can use I18n.with_locale (at least in the tests) and forget
about Globalize, which will make it easier to switch to Mobility in the
future.

spec/features/localization_spec.rb

@@ -3,7 +3,7 @@ require "rails_helper"
 describe "Localization" do
 
   scenario "Wrong locale" do
-    Globalize.with_locale(:es) do
+    I18n.with_locale(:es) do
       create(:widget_card, title: "Bienvenido a CONSUL",
                            description: "Software libre para la participación ciudadana.",
                            link_text: "Más información",