9.4.15-0+deb8u1) is not available, but a newer one is (9.4.18-0+deb8u1). We redirected you there.
/usr/lib/postgresql/9.4/bin/postmaster is in postgresql-9.4 9.4.18-0+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/postgresql/9.4/bin/postgres