CircleCI: use openjdk:9-jdk image to build instead of 8-jdk

The original (CirclecI 1.0) build used JDK 9. The Docker Hub UI only
lists 100 tags per repository, not showing version 9, so the initial
migration used JDK 8. The API confirms we get version 9 images:

curl \
  https://registry.hub.docker.com/v1/repositories/circleci/openjdk/tags \
  | jq '.[] | .name' -r | grep '^9'

Author: abesto

.circleci/config.yml

@@ -16,7 +16,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: circleci/openjdk:8-jdk
+      - image: circleci/openjdk:9-jdk
     steps:
       - checkout