Bump nokogiri to 1.8.4 and sanitize to 4.6.6 for performance
nokogiri 1.4.4 has this memory leak fix: https://github.com/sparklemotion/nokogiri/pull/1771
sanitize 4.6.6 has this optimization that will benefit Markdown rendering: https://github.com/rgrove/sanitize/pull/183
Edited by Stan Hu