Release v4.0.0 (googleapis#666)

Merge pull request googleapis#575 from jonparrott/cut-3.0.0

Release 3.0.0.

Merge pull request googleapis#519 from jonparrott/cut-2.2.0

Release v2.2.0.

Fixing changelog typo

Merge pull request googleapis#501 from jonparrott/cut-2.0.2

Release v2.0.2.

Merge pull request googleapis#449 from dhermes/cut-2.0.1

Release v2.0.1.

Merge pull request googleapis#423 from dhermes/cut-2.0.0-post1

Release v2.0.0-post1.

Merge pull request googleapis#404 from nathanielmanistaatgoogle/relea…

…se-2.0.0

Release 2.0.0.

Merge pull request googleapis#341 from thobrla/master

Release v1.5.2

Merge pull request googleapis#304 from dhermes/release-1.5.1

Release 1.5.1.