Update facter PR 617 - AIX netmask  (FACT-247)

This PR pulls in the changes that @jcitarello proposes in https://github.com/puppetlabs/facter/pull/617, with a correction to the location of 'AIX' in lib/facter/util/netmask.rb, per the discussion in https://github.com/puppetlabs/facter/pull/617. My changes are in aebc0bd, with @jcitarello's changes preceding. This PR closes https://github.com/puppetlabs/facter/pull/617.