Vertex Removal `ids`

[rosenbrockc]

The session objects pops the vertex hashable id from its current list; however, if the vertex was not first saved in the current session, then the id will not exist, so it cannot be popped. This is a trivial fix at https://github.com/davebshow/goblin/blob/master/goblin/session.py#L262:

        if hashable_id in self.current:
            vertex = self.current.pop(hashable_id)

Which branch should I PR to?

[rosenbrockc]

Same thing for edge removal:

        if hashable_id in self.current:
            edge = self.current.pop(hashable_id)

[davebshow]

Master will be fine.