Auto delete

Fixes #12 
If you change the code to be:

```ruby
Todo.destroy_all
```

this test will still pass.

When testing try to think about "what can go wrong here?" and "what mistakes I could make here in the future?" When you write just one test for a functionality it usually means there's something missing.

Write another test that checks that `DeleteTodos.run` doesn't remove tasks that are not old enough.