Don't use system gems when using system ruby

[cwolferh]

bootstrap.sh right now is rather fragile in "system ruby" mode. The problems tend to lie in the versions of gems that may already exist on the system, for instance, the oft-mentioned puppet bug that complains if a the puppet group does not exist. There has also been an issue with newer facter gems. And, rubygem-thor on Fedora 17 causes an issue later on during bundle install.

I think the solution is just to ignore system ruby gems, rather than try to the multitude of bugs we may face across distros / gem versions. In particular, this worked well for me on Fedora 17 with the "problem gems" already installed on the system.
cwolferh/dev-tools@master...less_fragile_system_ruby2

If there isn't an objection, I'll submit a pull request after #48 gets merged (they both touch one of the same lines in bootstrap.sh, so avoiding a merge conflict).