update libxml2 and nokogiri to fix japanese comment rendering on heroku

Hi

I fixed japanese comment rendering on heroku.

libxml2 and nokogiri caused rendering issue on heroku.
[ruby on rails - Nokogiri adds characters during parsing on Heroku - Stack Overflow](http://stackoverflow.com/questions/17980737/nokogiri-adds-characters-during-parsing-on-heroku)

This PR update libxml2(use buildpack) and nokogiri(use Rubygems).
I use buildpack-muliti becasuse libxml2 buildpack needs ruby buildpack.(only libxml2 don't work apps)

if you deploy on heroku, you should add buildpack config.

    $ heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
    $ git push heroku master