travis: Stop building go 1.6
travis: Stop building go 1.6

At this stage in the project, it's reasonable to assume the latest
version.  Plus builds will complete faster.

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