Bug #17898 - pkgdmg provider broken since version 2.7.13 under OS X 10.5...

... Leopard

http://projects.puppetlabs.com/issues/17898

Dir.mktmpdir doesn't exist under Ruby 1.8.6 or older. This adds it, using code from https://github.com/puppetlabs/hiera/pull/47/files which uses code from ruby-1.8.7-p357/lib/ruby/1.8/tmpdir.rb

Puppet isn't supported under 1.8.6, but it is under 1.8.5, so this fixes it for those users too. :-)