doc: jenkins-iojs.nodesource.com -> ci.nodejs.org

The Jenkins URL has changed from https://jenkins-iojs.nodesource.com
to https://ci.nodejs.org. The former address now redirects to the
latter. Some jobs has also changed names.

PR-URL: https://github.com/nodejs/node/pull/2886


Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Roman Reiss <me@silverwind.io>