Update lib/facter/version.rb for 1.6.18
Merge branch 'release/1.6.18' into 1.6.x

* release/1.6.18:
  Update lib/facter/version.rb for 1.6.18
Merge branch '1.6.x' into 1.7.x

* 1.6.x:
  Update lib/facter/version.rb for 1.6.18