Change webhook RBAC to talk about apps.deployments instead of v1beta1.deployments

[vaikas]

Problem

In the pkg/webhook we use extensions.v1beta1.deployments
but in our eventing/controller we use apps.v1.deployments

and that means that our deployments RBAC are little different between the two.

We should upgrade to latest pkg and then update these:
knative/pkg#345

Persona:
Which persona is this feature for?

Exit Criteria
once the rbac rules both use apps/deployments

Time Estimate (optional):
.2

Additional context (optional)
Add any other context about the feature request here.