Prep 'storage-1.5.0' release. (googleapis#4202)

Rolling back the dependency update for trace.

This is so that an "off-master" release can be done
without being gated on a `google-cloud-core` release.

Pub/Sub version bump: 0.28.4 (googleapis#4183)

Firestore: Remove incorrect markup from README.rst.

Move to google.cloud.spanner_v1 (googleapis#4064)

Bump video version number. (googleapis#3995)

Video Intelligence: Fix import issue (googleapis#3985)

Video Intelligence v0.27.0 (googleapis#3977)

Natural Language 0.29.0 (googleapis#3972)

Pubsub 0.28.3 (googleapis#3921)