Switch from oraclejdk9 to openjdk9 in Travis builds, per https://travis-ci.community/t/java-9-build-failing-with-https-certificate-exception/4364/2.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=263775386

Author: netdpb

.travis.yml

@@ -9,7 +9,7 @@ notifications:
 
 jdk:
   - oraclejdk8
-  - oraclejdk9
+  - openjdk9
   - openjdk10
   - openjdk11
   - openjdk-ea