C client: Fix up version numbers in release notes

Remove "GA" trailer from GA versions, and place version numbers for
previous two releases.
Merge pull request #155 from mnunberg/stage

C client: Fix up version numbers in release notes
Dotnet 1.3.9 release notes
Merge pull request #157 from jeffrymorris/dotnet-139

Dotnet 139
.NET SDK: edit release notes
Update release notes for Python SDK 1.2.3
Merge pull request #158 from mnunberg/stage

Update release notes for Python SDK 1.2.3
Python SDK: edit release notes