Check for puppet executable instead of gem. Additional fix for #38

Using my fix, I can run librarian-puppet on Windows with Puppet 3.3.1 installed from an MSI package. This is the first Puppet version on Windows that adds Puppet to the path by default.
http://projects.puppetlabs.com/issues/22700

This snippet comes from a comment of @patcon for commit dbe0163b577802652f4ed55de451435ec4b3d744 but with some cleanup to correctly ignore output on Windows.