sd/etcd now requires Go 1.9

[peterbourgon]

sd/etcd imports github.com/coreos/etcd/client, which as of this commit 11 days ago now requires Go 1.9 due to the context and aliasing blah-blah-blah. Sigh.

I judge the contortions required to keep Go kit master building on anything other than the latest stable release not worth the effort, especially in the era of reasonable dependency management for downstream consumers i.e. dep. Thus I'll change the relevant CI systems to test on go1.9 and tip only.