updating ohai dep to 2.0

Ohai is at version 2.0. The nginx cookbook still lists ~> 1.1 as a dependency.

Is there a particular reason to keep the dep anchored to 1.1?

Also, knife cookbook site install nginx will download the latest version of ohai (2.0) creating a 'wedged' install until the user deletes ohai and specifies version 1.1 or edits metadata.rb in the nginx cookbook.