This was broken because of a wrong tag being generated for 0.2.3.

The script scripts/changelog.rb generates a changelog from Git commits,
in a straightforward format. The make task "changelog" can be used to
trigger this. This means tagging a release now comes down to:

1. Update the various version files.
2. Run "make changelog" and commit the changes.
3. Run "make tag" to create and push a Git tag.

Fixes #133