Delegate t() to Rails t() instead of i18n.t()

Rails t() offers a couple more nice features before calling I18n.t()
see http://api.rubyonrails.org/classes/ActionView/Helpers/TranslationHelper.html#method-i-translate

about #3176

Unless we make Spree.t() behave the same way as Rails t() I think we should also remove the warnings telling users to stop using t() in favor of Spree.t(). What do you think?