Update pandoc.rb with pandoc 1.18.
Update pandoc.rb with pandoc 1.18.

Pandoc 1.18 changed the JSON output format, check
http://pandoc.org/releases.html#pandoc-1.18-26-oct-2016.

semantic-ui-forest/semantic-ui-forest,nanoc-plate/nanoc-plate,nanoc-plate/nanoc-plate,tarvos21/semantic-ui-forest,semantic-ui-forest/semantic-ui-forest,nanoc-plate/nanoc-plate,tarvos21/semantic-ui-forest,tarvos21/semantic-ui-forest,semantic-ui-forest/semantic-ui-forest,semantic-ui-forest/semantic-ui-forest,nanoc-plate/nanoc-plate,tarvos21/semantic-ui-forest