COOK-4651- xml cookbook depends on nokogiri, but nokogiri requires git

Hello! Please see the following bug report:
https://tickets.opscode.com/browse/COOK-4651#comment-49624

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