devpi-server-2.5.3: small fix preventing traceback on replica

Just did a devpi-server-2.5.3 which fixed a minor regression on replicas when they hit a non-existing project. best, holger 2.5.3 (2015-11-23) ------------------ - fix a bug that resulted from accessing a non-existing project on root/pypi where upstream does not contain the X-PYPI-LAST-SERIAL header usually. Thanks Matthias Bach.