Our `rsync` package was shipped with bundled `zlib` to provide compatibility with the old-style `--compress` option up to version 3.1.0. Version 3.1.1 was released on 2014-06-22 and is shipped by all major distributions now.
So we decided to finally drop the bundled library and ship a package with system `zlib`. This also fixes security issues, actual ones and in future. Go and blame those running old versions if you encounter errors with `rsync 3.1.3-3`.