Update faas gateway to 0.12.0

0.12.0 moves the Prometheus port to 8082 (internal) and not
exposed on the network

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

docker-compose.yml

@@ -3,7 +3,7 @@ services:
     gateway:
         ports:
             - 8080:8080
-        image: openfaas/gateway:0.11.1
+        image: openfaas/gateway:0.12.0
         networks:
             - functions
         environment: