Release google-cloud-storage 1.15.0
- Add Bucket retention policy
- Add retention_policy fields and default_event_based_hold to Bucket.
- Add retention_policy and hold fields to File.
- Add Bucket#lock_retention_policy!
- Add Bucket#metageneration.
- Add Bucket#retention_policy_locked?
- Add File#(set|release)_temporary_hold!
- Add File#(set|release)_event_based_hold!