Update redcarpet to 2.3.0

2.3 has [fixed a few things](https://github.com/vmg/redcarpet/compare/v2.2.2...v2.3.0) (the [smart single quote](https://github.com/vmg/redcarpet/commit/0eec7ac25d0f553903c46254e772893f2386ef99) but it still has support Ruby 1.8.x.