Prepare v0.43.0 (googleapis#736)

Prepare v0.42.0 (googleapis#728)

Prepare v0.41.0 (googleapis#722)

* Prepare v0.41.0

* Don't report code coverage on hhvm

* Update google/gax and google/proto-client dependency versions

Prepare v0.40.0 (googleapis#705)

Prepare v0.39.2 (googleapis#678)

Prepare v0.39.1 (googleapis#671)

Prepare v0.39.0 (googleapis#663)

Prepare v0.38.0 (googleapis#659)

* Prepare v0.38.0

* Address code review

Prepare v0.37.0 (googleapis#653)

Prepare v0.36.0 (googleapis#612)