.travis.yml: fix possible issues

* use `pool.sks-keyservers.net` as keyserver as it's often more reliable
than `keyserver.ubuntu.com` as you can see with Google.
* put script to multiple lines
    * build out-of-source
    * use three CPUs
        * make seems to fail if $(nproc) is used unlike WeeChat. ZNC has
          same issue.
* mark build as failed if one job fails immediately without waiting for
all builds to finish.
    * this doesn't stop jobs that didn't finish before build was marked
    failed.