Don't require a name for a subscription

The [Pub/Sub documentation](https://cloud.google.com/pubsub/reference/rest/v1/projects.subscriptions/create) states:

> If the name is not provided in the request, the server will assign a random name for this subscription on the same project as the topic.

\- https://github.com/GoogleCloudPlatform/gcloud-ruby/issues/193 / @blowmage
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So, let's not require it!
