Update installation.md - Updated the Ruby section.
Update installation.md - Updated the Ruby section.

Fixes my previous my old obsolete/forgotten pull request / patch (sorry about that):
#6262
https://github.com/lpalgarvio/gitlabhq/tree/patch-1
axilleas/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,iiet/iiet-git,axilleas/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,dreampet/gitlab,mmkassem/gitlabhq,iiet/iiet-git,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,dreampet/gitlab