`create-github-release.js` had an implicit assumption that it was run
before the new version was tagged. It would generate the change log
based on PRs merged since the most recent tag. However Jenkins now runs
this script after the tag is created. As a result the change log was
always empty.

Fix the issue by explicitly passing the previous version as an argument
to the script so that it doesn't matter whether the new version has been
tagged or not.