Use facter 3 facts during tests

By default, rspec-puppet-facts uses the facterdb facts that match the
version of the facter gem installed. Starting with facter 3, facter
was no longer shipped as a gem (it was rewritten in C++).

This commit sets facterversion to 3.0.0 so that tests run with
more modern (structured) facts stubbed.

Puppet has been shipping facter 3 since puppet 4.2.0. This module
supports puppet >= 4.6.1, (and puppet 4 is EOL at the end of 2018).

puppet-extlib's ip_to_cron function (used in
puppet::agent::service::cron) has recently moved to using the
networking structured fact not available in facter 2.