Unknown Component: apex:emailPublisher

I'm trying to write a simple VF page using below code:

<apex:page standardController="Case"><apex:emailPublisher entityId="{!case.id}"                         toAddresses="{!case.contact.email}"/></apex:page>

When I try to save, I receive the error:

Error: Unknown component apex:emailPublisher

• I'm using API Version 39.0, and documentation shows this came-out in API=25.
• I have Case Feed enabled and working.
• None of the Case Feed components are showing the VF editor