Prepare v0.28.0

[dwsupplee]

Google Cloud PHP v0.28.0

Cloud Vision v0.3.0

• A GAPIC generated layer for vision has been added. (Add vision GAPIC #463)

Cloud Logging v1.1.0

• Support for insertId and timestamp on Logger::entry(). ([BC Break] Add timestamp/insertId support to Logger::entry() #462)

Cloud PubSub v0.4.0

• ❗ BREAKING CHANGE The Google\Cloud\PubSub\Duration and Google\Cloud\PubSub\Timestamp classes have been moved out of PubSub and into Core to be shared. Subsequently, PubSubClient::timestamp() and PubSubClient::duration() now return Google\Cloud\Core\Timestamp and Google\Cloud\Core\Duration instances. ([BC Break] Add timestamp/insertId support to Logger::entry() #462)
• Shoutout to @adriankirchner for addressing a doc issue in the publish methods. ([Docs] Fix incorrect Pub/Sub example resulting in BadRequestException #460)

Cloud Core v1.2.0

• Google\Cloud\Core\Duration and Google\Cloud\Core\Timestamp classes have been added. ([BC Break] Add timestamp/insertId support to Logger::entry() #462)
• A fix for an issue with handling sub second precision while using gRPC. ([BC Break] Add timestamp/insertId support to Logger::entry() #462)