use Gem::Version instead of Chef::Version

The Chef::Version class is only for cookbook versions and only supports
x.y.z version numbers.  It cannot be used to compare chef rubygems
versions without completely breaking prereleases.

See:

https://github.com/opscode/chef/blob/cb978d59239e7fcfea48749334185caa7064b9ea/lib/chef/version.rb#L23-L30