Upgrade cloudevents-go to the new SDK

[mattmoor]

The cloudevents SDK for Go has been getting overhauled, and we should update the Serving sample cloudevents-go to match.

/assign @slinkydeveloper @n3wscott
cc @duglin

[n3wscott]

I can do this... I have a few things I want to show people.

[slinkydeveloper]

@n3wscott cool, loop me in the review ;)