travis: Add slack notifications
travis: Add slack notifications

Haven't tested this, but hopefully it will automatically post to the
quilt slack channel.

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