Update rubygems
https://www.trustwave.com/Resources/Security-Advisories/Advisories/TWSL2015-009/?fid=6478
gem install
is vulnerable to MITM attacks via DNS poisoning.
We install rubygems during the build in https://github.com/chef/omnibus-software/blob/master/config/software/rubygems.rb ; we can override the version and md5 in config/projects/gitlab.rb.