refactors localization spec using expect instead of global state

spec/features/localization_spec.rb

@@ -34,13 +34,10 @@ feature 'Localization' do
   end
 
   scenario 'Locale switcher not present if only one locale' do
-    initial_locales = I18n.available_locales
-    I18n.available_locales = [:en]
+    expect(I18n).to receive(:available_locales).and_return([:en])
 
     visit '/'
     expect(page).to_not have_content('Language')
     expect(page).to_not have_css('div.locale')
-
-    I18n.available_locales = initial_locales
   end
 end