bug#27294: R importer fails to create a list for multiple licenses

[Joshua Sierles]

For example, 'guix cran import RgoogleMaps' generates this line:
(license (gpl2+ gpl3+)). Build fails unless changing the line to: 
(license (list gpl2+ gpl3+)).

-- 
  Joshua Sierles
  jos...@joshua.si