travis: No longer depend on docker
travis: No longer depend on docker

We're going to migrate to the docker build system for our containers,
so we no longer need to depend on docker in travis.

kklin/quilt,ZhongtianWang/quilt,kklin/kelda,akshayknarayan/quilt,tejasmanohar/quilt,quilt/quilt,secant/quilt,NetSys/quilt,kklin/kelda,kklin/kelda,LuiseV/quilt,kayousterhout/quilt,kklin/quilt,ejj/quilt-dev,kayousterhout/quilt,ejj/quilt-dev,kklin/quilt,LuiseV/quilt,PenguinToast/quilt,kklin/kelda,secant/quilt,kelda/kelda,ZhongtianWang/quilt,NetSys/quilt,quilt/quilt,secant/quilt,quilt/quilt,stevenlin598/quilt,LuiseV/quilt,kayousterhout/quilt,ZhongtianWang/quilt,mmussomele/quilt,akshayknarayan/quilt,kayousterhout/quilt,NetSys/quilt,PenguinToast/quilt,kklin/quilt,tejasmanohar/quilt,akshayknarayan/quilt,quilt/quilt,mmussomele/quilt,kelda/kelda,secant/quilt,stevenlin598/quilt,mmussomele/quilt,PenguinToast/quilt,mmussomele/quilt,LuiseV/quilt,mmussomele/quilt,PenguinToast/quilt,kelda/kelda,stevenlin598/quilt,ejj/quilt-dev,mmussomele/quilt,kelda/kelda,stevenlin598/quilt,ZhongtianWang/quilt,stevenlin598/quilt,stevenlin598/quilt,NetSys/quilt,ejj/quilt-dev,akshayknarayan/quilt,tejasmanohar/quilt,tejasmanohar/quilt,NetSys/quilt,secant/quilt,PenguinToast/quilt,LuiseV/quilt