Update `migratus.core/rollback` behavior to roll back last applied migration when no arguments are provided

[S4G4R]

The documentation states : Run 'down' for the last migration that was run.

Which would mean it should roll back the last migration that was applied.

Instead, I find that it rolls back according to the id (migration creation timestamp).

Reproduction Steps :

1. Create 3 migrations one after the other (say A -> B -> C)
2. Apply migration A first, followed by C and then B
3. Run migratus.core/rollback

Result :

• Expected order : B -> C -> A
• Actual order : C -> B -> A

[yogthos]

Yeah, the documentation is misleading there. I'll update the docs, but I'd be open to switching the behavior to running the last applied migration. Would you be up for doing a PR for this by any chance? :)

[S4G4R]

Still relatively new to Clojure, but I will give it a go! :)

[yogthos]

Sounds like a plan, and I'll try assist time permitting. :)