Fix "jekyll build" RuntimeError

[tonyduckles]

Fix a jekyll build RuntimeError, when using the latest version of this gem (1.7.0) and and jekyll (3.7.2).

/srv/bundle/bundler/gems/codefence-1ff084fe35c8/lib/octopress-codefence.rb:26:in `encode!': can't modify frozen String (RuntimeError)
        from /srv/bundle/bundler/gems/codefence-1ff084fe35c8/lib/octopress-codefence.rb:26:in `render'
        from /srv/bundle/bundler/gems/codefence-1ff084fe35c8/lib/octopress-codefence.rb:12:in `block in <module:Codefence>'
        from /srv/bundle/gems/jekyll-3.7.2/lib/jekyll/hooks.rb:103:in `block in trigger'
...

Extends off of #14.