Fix frontend features locale_spec

It failed with:

     ActionView::Template::Error:
       undefined method `content_tag' for Spree:Module

Probably due to recent changes in rails i18n api related to the security
patch in 4.0.2, see https://groups.google.com/forum/#!topic/ruby-security-ann/pLrh6DUw998
and rails/rails@78790e4

core/lib/spree/i18n.rb

@@ -4,6 +4,7 @@
 
 module Spree
   extend ActionView::Helpers::TranslationHelper
+  extend ActionView::Helpers::TagHelper
 
   class << self
     # Add spree namespace and delegate to Rails TranslationHelper for some nice