Allow puppet-ruby to run on hosts without Boxen facts

A couple of small tweaks that makes this module run on non-mac hosts:

* Only include the boxen homebrew path in the rubygems provider if `boxen_home` fact is present
* Only try to run the gem commands as the boxen user if `boxen_user` is present
* Generate `rbenv.sh` from a template, rather than hardcoded file