Always install 'librarian-puppet' as a gem in apt-based systems

The librarian-puppet packages for `apt` install old versions of the lib (check https://packages.qa.debian.org/libr/librarian-puppet.html and http://packages.ubuntu.com/search?suite=default&section=all&arch=any&keywords=librarian-puppet&searchon=names for more details), let's go with `gem`!

Also, this PR adds the ruby1.9.1-dev packages (required by `gem` installer) and checks if the ruby-json package is not installed in order to avoid unrequired system changes.