.sync modifier is valid since v2.3.0+

[laander]

I realize that this project might not be maintained anymore, but i wont to mention that the v-bind-sync warning throws a false-positive as it has actually been reintroduced into Vue2.

Please see: https://vuejs.org/v2/guide/components-custom-events.html#sync-Modifier

[chrisvfritz]

We should actually still probably warn about the .sync modifier, because it's behavior is different than in 1.x and will still require a refactor to emit the proper event from the child component. A PR to update the message would be very welcome!