Add downgrade info

Today I've faced very weird behaviour such as

```
mail: mikel/mail@2.5.4...2.2.7
```

mail gem is downgraded unexpectedly.

As this super handy library, it's easy to find, and even more, if there's informaitive text might be useful.

```
mail: mikel/mail@2.5.4...2.2.7 (downgrade)
```

I'm really sorry about lack of test :bow: