Update gem installation instructions.

- Remove `gem install redcarpet`. Redcarpet was an external dependency
  until Jekyll 1.1.0 was released 2013-07-14.
- Add `gem install RedCloth`. RedCloth is required for Textile
  templates in _includes/disqus.html and _layouts/badges.html.
Merge pull request #319 from goxberry/remove-redcarpet-gem-from-readme

Remove `gem install redcarpet` from README.md.