Add slimmer to Dependabot config
Add slimmer to Dependabot config

This got missed in the initial config creation.
As Slimmer is an internal gem, it should be added
to the config.

https://trello.com/c/uPoriyfJ/2049-add-dependabot-configuration-to-each-repo-blitz-pair

alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers