"validatePresence on" not updating

`validatePresence({ presence: true, on: 'email' })`

## Version
2.1.0

This is probably me misunderstanding the behavior of the validator.

## Steps to reproduce

```
export default {
email: [
validatePresence(true),
validateFormat({ type: 'email' }),
],
password: [
validatePresence({ presence: true, on: 'email' }),
],
}
```

## Expected Behavior
Empty password validates fine, but

1. turns invalid once an email is entered, or
2. turns invalid after focused after email is entered.

## Actual Behavior

Empty password stays valid, focused or not.
Once you type in a password _and then remove it again_ then the validator fails (as expected).