throw exception when event_type is push and the field deleted is true

[KeiichiHirobe]

GitHub sends two events when refs are deleted. One is Delete event and the other is Push event the field deleted of which payload is true. And the field head_commit of Push events payloads is null.

So, we see Unsupported GitHub event: 'delete' and 'NoneType' object is not subscriptable messages when git refs are deleted, and I think the latter message is a little surprising. I added a change to check whether the deleted field is true and skips parsing logic if true.