Gentoo facter 1.7 support

Facter 1.7 added Gentoo as its own osfamily, but this module currently assumes that the osfamily on Gentoo is 'Linux' which means an error will be displayed on an up to date Gentoo machine.

This commit detects Gentoo as its own osfamily for Facter 1.7 but leaves the fallback to osfamily 'Linux' if Facter 1.6.x is still in use.

Fixes GH-83.