Add a facter fact for dist-upgrade

[willmeek]

This commit adds a facter fact for dist-upgrade, along with
associated facter facts.

(Builds upon PR #634 - adds a new set of facts to maintain compatibility)

[eputnam]

i took a peek at tests. may have something to do with scoping in RSpec. while i was testing apt_dist_has_updates_spec.rb, the fact was returning nil, not even false for the last example but putting File.expects(:executable?).with('/usr/bin/apt-get').returns true in the actual example (instead of the before block), it seemed to help a little.

[willmeek]

@eputnam I can't replicate this, if I add the expect outside of the block then it returns nil, the opposite of what you are seeing. Travis has gone green too. Let's discuss this later.

[eputnam]

there's a Facts section in the readme, would you mind updating that?

[willmeek]

Facts in readme updated, thanks

[willmeek]

Thanks @eputnam