--no-ri --no-rdoc is deprecated, use --no-document

/ruby/gemrc.symlink :
this congifuration was using 'gem: --no-ri --no-rdoc' to avoid
installation of docs with gems, but this method is now deprecated
changed it to new official way => 'gem: --no-document'
Merge pull request #157 from pravj/update-gemrc

--no-ri --no-rdoc is deprecated, use --no-document