Update download pages at latest resources.

I updated download pages. following topics.

 * Download links and ruby-lang.org url were replaced by https.
 * (en) Added lists of download mirror.
 * (ja) Remove too old links.
 * (ja) Remove download links of Ruby 1.8

@stomar @take @kentaro Please check these grammars and fit for the topics.
(@kentaro is my colleague :smile: )