Use redcarpet (Github Flavored Markdown) because Maruku breaks pages

## background

- In scalajp/scala.github.com#29 @xuwei-k reported that Sets and Seqs are no longer visible in Japanese translation of the Collections overview.
- In scala/scala.github.com@4bfc706c3f7f6 @heathermiller confirmed that after gh-pages upgrading its jekyll the site no longer built, so the files were renamed to *.disable.

## what this changes

First, I am restoring the files to the original state.

Second, I'm changing the markdown processor to **redcarpet** (Github Flavored Markdown). The bug seems to be coming from Maruku's handling of non-latin characters, and this is not the only time I've seen it break. I pretty much avoid any use of list or tables because Maruku is so bad. (If you or @xeno-by was wondering why I'm always using html in the translations, this is the reason)

With [GFM][1] comes fensed code block and all the goodies that we're used to in the rest of github ecosystem.
If it's so happy and shiny, why isn't it turned on by default? I don't know, but maybe because they've been  developing redcarpet in more recent Ruby environment, and you have to take a few steps to set it up locally.

## github/pages-gem

You can see what gh-pages is using with [github-pages.gemspec][2].

- Ruby 2.0.0
- jekyll 1.1.2
- redcarpet 2.2.2

Or `gem install github-pages` says [Using Jekyll with Pages](https://help.github.com/articles/using-jekyll-with-pages).

## demo

I merged this into scalajp/scala.github.com's gh-pages branch to make sure it works:

- http://scalajp.github.io/scala.github.com/ja/overviews/collections/seqs.html
- http://scalajp.github.io/scala.github.com/ja/overviews/collections/sets.html

Most of the links are broken since they assume toplevel `/`, but you can check the renderings by putting in `scala.github.com/` manually.

  [1]: https://help.github.com/articles/github-flavored-markdown
  [2]: https://github.com/github/pages-gem/blob/master/github-pages.gemspec