[WIP] Upgrade to jekyll2

Upgrading to Jekyll 2.x version.

@stomar confirmed to no html diffs. but It's not work with heroku environment yet. I'm investigating this issue.

ref. https://github.com/ruby/www.ruby-lang.org/commit/eedb3cbcc58ce3f5c00d8fbfef36dc52ec2695f0#commitcomment-8399630