[COOK-4651] pin nokogiri to a working version


Hello! Please see the following bug report:
https://tickets.opscode.com/browse/COOK-4651

This pull request pins nokogiri at a known, working version. Having nokogiri break as a surprise is very bad for autoscaling with chef. Also, this PR includes updates to resolve rubocop complaints.

Thank you!

    @MartinB3