ﾕｰｻﾞｰﾃﾞｨﾚｸﾄﾘにruby1.9.3をｲﾝｽﾄｰﾙしたなら環境変数でPATH指定しましょう｡
http://stackoverflow.com/questions/17204152/nokogiri-error-when-running-bundle-install

Turned out I was using the default system Ruby (and thus installing all the gems there in /Library/Ruby/Gems) even though rbenv points to ruby-1.9.3 so that's why Nokogiri was complaining.

Fixed this by adding this to $PATH, so rbenv loads before the system ruby:

export PATH="$HOME/.rbenv/bin:$PATH"