Update definitions/mongodb.rb

This is a fix for bug 89 (https://github.com/edelight/chef-mongodb/issues/89) given a breaking change made between chef 10 and 11 -- see https://github.com/edelight/chef-mongodb/issues/89#issuecomment-13646421 for details. This fix will work in both chef 10 and 11.
include?'s equality test is not reliable for Chef::Node objects