Update definitions/mongodb.rb

This is a fix for bug 89 (https://github.com/edelight/chef-mongodb/issues/89). The bug was caused by a breaking change to chef itself between 10 and 11 -- see http://docs.opscode.com/breaking_changes_chef_11.html#single-notifies-for-notification for details. This fix will work in both chef 10 and 11.