chore(rootfs/Dockerfile): update go toolchain to 1.9.1

See https://github.com/golang/go/issues?q=milestone%3AGo1.9.1

Author: mboersma

rootfs/Dockerfile

@@ -1,7 +1,7 @@
 FROM quay.io/deis/base:v0.3.6
 
 ENV AZCLI_VERSION=2.0.14 \
-    GO_VERSION=1.9 \
+    GO_VERSION=1.9.1 \
     GLIDE_VERSION=v0.12.3 \
     GLIDE_HOME=/root \
     HELM_VERSION=v2.6.0 \