Pin version of systemu to the same version included in chef.

systemu versions between chef and ohai is conflicting on windows now: 

C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/specification.rb:1637:in `raise_if_conflicts': Unable to act
vate ohai-6.20.0.rc.0, because systemu-2.5.2 conflicts with systemu (~> 2.2.0) (Gem::LoadError)
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/specification.rb:746:in `activate'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:51:in `block in require'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:50:in `each'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:50:in `require'
        from C:/opscode/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0.rc.0-x86-mingw32/lib/chef/http/http_request.
b:32:in `rescue in <top (required)>'
        from C:/opscode/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0.rc.0-x86-mingw32/lib/chef/http/http_request.
b:29:in `<top (required)>'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
        from C:/opscode/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0.rc.0-x86-mingw32/lib/chef/http/basic_client.
b:26:in `<top (required)>'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
        from C:/opscode/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
        from C:/opscode/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.8.0.rc.0-x86-mingw32/lib/chef/http.rb:26:in `<to
 (required)>'