Use Mongoid::Config.models to get the list of all available models in mongoid_slug.rake

[syzspectroom]

this PR will fix #247.
My proposal is to use Mongoid::Config.models (https://www.rubydoc.info/github/mongoid/mongoid/Mongoid/Config#models-instance_method) to get the list of all available models instead of iterating through all Module constants and check if Mongoid::Slug is included in the model using const.ancestors.include?(Mongoid::Slug) which is much safer to use.
This fix will prevent the task from failing with errors like this:

TypeError: compared with non class/module

[mongoid-bot]

	1 Warning
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

Generated by 🚫 danger

[dblock]

Thanks, we just need a green build. I think this method didn't exist in older mongoid versions so you have to do some ifs. Changelog too pls.

It would be nice to have a test that reproduces this problem.

[kailan]

This change is implemented in #255.