Fix warning on nil comparison

This uses the more efficient /regex/.match?(value) syntax that also
doesn't raise any warning if id is nil.

Author: ekohl

lib/fog/vsphere/compute.rb

@@ -347,7 +347,7 @@ def managed_obj_id(obj)
         end
 
         def is_uuid?(id)
-          !(id =~ /[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}/).nil?
+          /[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}/.match?(id)
         end
       end