fix download URL for R

Signed-off-by: Fotis Georgatos <fotis.georgatos@uni.lu>
Merge pull request #251 from fgeorgatos/bugfix_R_download_URL

fix download URL for R
Merge branch 'v1.4.0.x' of github.com:hpcugent/easybuild-easyconfigs into v1.4.0.x