travis: Upgrade to the latest version of go
travis: Upgrade to the latest version of go

This patch changes travis so it always builds with the latest
available version of go.  As a side effect, we should start building
with go 1.8 immediately.

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