Nokogiri Major Version Dependency

Changes to allow the installation of the plugin with the latest version of Vagrant which is bundled with Nokogiri 1.6.x. The dependency would allow all minor versions of Nokogiri after 1.5 to used. 
Why do you use this instead of optimistic dependency `>=1.5.10`?
That would allow reusing version 1.6 already loaded by Vagrant  core.