(PUP-1821) Require Facter 1.7 or greater

We're starting to rely on facts that were released in Facter 1.7.0,
which means that we always need to pull in 1.7 to make those facts
available.
Merge pull request #2405 from adrienthebo/issue/master/pup-1821-depend_on_facter_1.7

(PUP-1821) Require Facter 1.7 or greater