Deprecation: Document#ext is now a key in the #data hash

[laomo]

Deprecation: Document#ext is now a key in the #data hash.
Called by /Library/Ruby/Gems/2.0.0/gems/octopress-codefence-1.7.0/lib/octopress-codefence.rb:8:in `block in module:Codefence'.

[debugly]

use extname rather than ext.