Update DockerHub build hook with golang 1.9 (#998)

To align with #992
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

hooks/pre_build

@@ -3,5 +3,5 @@ echo "=> Building the CoreDNS binary"
 docker run \
   -v $(pwd):/go/src/github.com/coredns/coredns \
   -w /go/src/github.com/coredns/coredns \
-  golang:1.8.0 \
+  golang:1.9 \
   make