Allow prereleases to be included in the chefdk

The rubygems code is here: https://github.com/rubygems/rubygems/blob/3e365282d1fa4210011b3095d685c10483a7a62c/lib/rubygems/specification.rb#L904-L922

You'll have to dig a bit deeper into `Gem::Dependency.new(name, *requirements).to_spec` to figure out how it decides whether to allow prereleases to see if there's a more elegant way to do this.