def page_should_display_sign_in_error
    page.body.should include(
      I18n.t('flashes.failure_after_create', :sign_up_path => sign_up_path)
    )
  end

is giving an error:  

 1) Visitor signs in tries with invalid email
     Failure/Error: page.body.should include(
     NoMethodError:
       undefined method `should' for #<String:0x007fa5301fad88>