Update version to 2.21.0

Add release notes for 2.21.0

Update version to 2.20.0

Update CircleCI xcode version (Automattic#3006)

This updates the CircleCI Xcode image to the latest stable version.

Add 2.20.0 release notes

Update CircleCI xcode version (Automattic#3006)

This updates the CircleCI Xcode image to the latest stable version.

Update Simperium dependency to latest and version to 2.19.0-beta2

Merge remote-tracking branch 'origin/develop' into release/2.19.0

Update version to 2.18.0

Add release notes for 2.18.0