The logs at https://travis-ci.org/servo/saltfs/jobs/87759990, specifically

    The following packages have unmet dependencies:
     salt-minion : Depends: salt-common (= 2015.5.3+ds-1precise2) but it is not going to be installed
                   Depends: python-zmq (>= 13.0.0) but it is not going to be installed
                   Recommends: dmidecode but it is not going to be installed
    E: Unable to correct problems, you have held broken packages.
    The command ".travis/install_salt" failed and exited with 100 during .
    Your build has been stopped.

make me think that having those packages available on the system will allow the
build to continue. Maybe.