Rouge's Redcarpet plugin now has a different API

See for more:
- https://github.com/jneen/rouge/commit/18ec938e9ca9463fb424c3d706f8fc761dad2460
- https://github.com/jneen/rouge/issues/182
Latest version of Rouge so we know the API is right.
Merge pull request #2951 from jekyll/fix-rouge