Switch from using Chef::Version to Gem::Version

ChefDK uses prerelease versions, which don't follow the strict filtering
that Chef::Version uses for cookbooks. As a result, trying to use this
plugin with a chefdk knife causes a failure. For more detail, please see:

https://github.com/opscode/chef-dk/issues/32
Merge pull request #21 from stormerider/master

Switch from using Chef::Version to Gem::Version

Hi all,

Sorry for the delay in merging this, I'm not doing much work with Chef at the moment.

I'll approve it now.

Matt