Removed protoc-artifacts/target directory

I committed this directory by mistake and these build artifacts should
not be checked in.

Update protobuf version

Update protobuf version

Update protobuf version

Update version number to 3.13.0.1 for php (protocolbuffers#7950)

Update protobuf version

Update protobuf version

Updated version to 4.0.0-rc2

Updated CHANGES.txt to reference the new version 4.0.0-rc1.

Update protobuf version