Remove Go1.6 support (#1492)

vet.sh

@@ -63,7 +63,6 @@ trap cleanup EXIT
 git ls-files "*.go" | xargs sed -i 's:"golang.org/x/net/context":"context":'
 set +o pipefail
 # TODO: Stop filtering pb.go files once golang/protobuf#214 is fixed.
-# TODO: Remove clientconn exception once go1.6 support is removed.
 go tool vet -all . 2>&1 | grep -vF '.pb.go:' | tee /dev/stderr | (! read)
 set -o pipefail
 git reset --hard HEAD