Added conditions for the cases in which RubyGems does not come by default with older versions of Ruby (resolves the issue discussed in #843).
Audit: improve messages for `RubyGems` (fix #843)
Treat the cases in which, on some OSes, `RubyGems`
does not come by default with older versions of `Ruby`
Looks fine, but do we need it? I thought we were only supporting modern rubies which have rubygems by default?
@tomlane We have to support Ruby 1.8.7 since that's the most common default on OSes...