Nokogiri needs to be updated

There was a security release for Nokogiri a few days ago and bundler audit is now failing on master because of that.

I've started working on the upgrade but:

• gollum-lib depends on Nokogiri ~> 1.6.4 but Nokogiri 1.7.0 drops the support of Ruby 1.9.2, 1.9.3, and 2.0.0 and gollum-lib advertise its support of Ruby 1.9.3
  • I've opened https://github.com/gollum/gollum-lib/pull/243 to update the Gem dependency
  • Wait for a 5.x release of gollum-lib: https://github.com/gollum/gollum-lib/milestone/3
• omniauth-cas3 depends on Nokogiri ~> 1.6.6
  • I've opened https://github.com/tduehr/omniauth-cas3/pull/2 to update the Gem dependency
  • Wait for a new release of omniauth-cas3
• truncato needs to be updated to 0.7.9 since 0.7.8 depends on Nokogiri ~> 1.6.1

/cc @rspeicher @briann