Fix i18n and nokogiri vulnerabilities

https://www.openproject.org/work_packages/7160

This is #1150 against release/3.0 (I created #1150 incorrectly against dev).


i18n: [0.6.8 is lowest 0.6.x version available](https://rubygems.org/gems/i18n), [Changelog](https://github.com/svenfuchs/i18n/commits/61f975bd862e2ce3a9f77fb5afdf11831fbcbb22)

nokogiri: [only had bugfixes](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.rdoc#1511--2013-12-14)