[COOK-4651] Pin nokogiri at a working version

The xml community cookbook has a dependency on nokogiri [1]. However, nokogiri just did a new release [2], and it uses a new version of mini_portile [3], which has a brand new dependency on 'git apply' being a working command.

I have notified the nokogiri project [4], but I also think it's important to pin the xml cookbook to known working versions of dependencies (instead of 'any nokogiri version you can find').

[1] https://github.com/opscode-cookbooks/xml/blob/master/recipes/ruby.rb#L35
[2] https://groups.google.com/forum/#!topic/nokogiri-talk/dPtmByhszRA
[3] flavorjones/mini_portile#32
[4] sparklemotion/nokogiri#1102

recipes/ruby.rb

@@ -32,4 +32,6 @@
 # See https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.rdoc#160rc1--2013-04-14
 ENV['NOKOGIRI_USE_SYSTEM_LIBRARIES'] = node['xml']['nokogiri']['use_system_libraries'].to_s
 
-chef_gem 'nokogiri'
+chef_gem 'nokogiri' do
+  version '1.6.2.1'
+end