I don't know why nokogiri wouldn't load. I didn't install any other gems, or change my ruby version.
I reinstalled and it seems to be working. But I got some warnings... Just a head up.
irb(main):001:0> require 'nokogiri'
LoadError: Cannot load specified object - /usr/local/lib/ruby/gems/2.0/gems/nokogiri-1.6.1/lib/nokogiri/nokogiri.so
from /usr/local/lib/ruby/2.0/rubygems/core_ext/kernel_require.rb:45:in `require'
from /usr/local/lib/ruby/2.0/rubygems/core_ext/kernel_require.rb:45:in `require'
from /usr/local/lib/ruby/gems/2.0/gems/nokogiri-1.6.1/lib/nokogiri.rb:28:in `<top (required)>'
from /usr/local/lib/ruby/2.0/rubygems/core_ext/kernel_require.rb:110:in `require'
from /usr/local/lib/ruby/2.0/rubygems/core_ext/kernel_require.rb:110:in `rescue in require'
from /usr/local/lib/ruby/2.0/rubygems/core_ext/kernel_require.rb:35:in `require'
from (irb):1
from /usr/local/bin/irb20:12:in `<main>'
$ sudo gem20 install nokogiri
Building native extensions. This could take a while...
Successfully installed nokogiri-1.6.1
Parsing documentation for nokogiri-1.6.1
unable to convert "\x8C" from ASCII-8BIT to UTF-8 for lib/nokogiri/nokogiri.so, skipping
1 gem installed
irb(main):001:0> require 'nokogiri'
WARNING: Nokogiri was built against LibXML version 2.8.0, but has dynamically loaded 2.9.0