add support for LinuxMint operating system

Since [facter 2.1.0](https://docs.puppetlabs.com/facter/2.1/release_notes.html), [LinuxMint is now supported](https://docs.puppetlabs.com/facter/2.1/release_notes.html#improvements-to-operating-system-detection) by the `operatingsystem` and `osfamily` facts. Thus the os detection stuff needs to be updated.