Update installation.md

12:08 < mumm_ra> Am I supposed to use ruby 1.9.2 as shown ^^
12:09 < mumm_ra> Gem::InstallError: gem-content requires Ruby version >= 1.9.3.
12:09 < mumm_ra> An error occurred while installing gem-content (1.0.0), and Bundler cannot continue.
12:09 < mumm_ra> Make sure that `gem install gem-content -v '1.0.0'` succeeds before bundling.
12:09 < mumm_ra> when I bundle install as suggested
12:09 < mumm_ra> bundle install
12:11 < mpapis> mumm_ra, yep please use newer ruby, open a ticket to update this doc