docs: update link to iojs+release ci job

We recently sandboxed the release CI jobs to their own Jenkins instance
This commit updates the links found in `doc/releases.md` to point
people in the right direction.

PR-URL: https://github.com/nodejs/node/pull/5632


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>