3.0: Rename validatePresence to requirePresence.

The name of this method should be what it does, requiring the presence.
Everything is *validation* here (even other rules), but the requiring (or not) is the specific task of this method.

It is also closer to the former way of saying `require => true/false`.

The former method name stays as alias for BC reasons during upgrades until 3.0 stable.
"The message to show if the field presence validation fails."  Would be more appropriate imo.