Merge pull request square#735 from square/zachklipp/changelog-version…

…s-v0.21.2

Update version numbers and changelog for 0.21.2.

Releasing v0.21.1.

Merge pull request square#701 from square/zachklipp/changelog-0.21.0

Update changelog and version for v0.21.0 release.

Merge pull request square#554 from square/zachklipp/prep-v0.20.0

Releasing v0.20.0.

Releasing v0.19.0.

Releasing v0.18.0.

Releasing v0.17.3.

Releasing v0.17.2

Releasing v0.17.1.

Releasing v0.17.0.