The Kythe release at https://github.com/google/kythe/releases/download/v0.0.21/kythe-v0.0.21.tar.gz is linked against libmemcached10. This breaks on Debian Jessie (and presumably some other systems), where libmemcached10 is unavailable - only libmemcached11 is present. We should have an official release that uses libmemcached11.
The older version will still be relevant for other users (e.g. those on Debian Wheezy) so this should be an addition, not a replacement.