.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 as many CPUs as possible
        * this might cause some builds to fail. ZNC has had this issue,
        there haven't been any issues with WeeChat as far as I know.
        WeeChat uses cmake, ZNC doesn't but I am not sure if it affects
        anything.
* 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.