[skip ci] Travis: switch to openjdk8

Author: colinsurprenant

.travis.yml

@@ -17,5 +17,5 @@ matrix:
  fast_finish: true
 install: true
 script: ci/build.sh
-jdk: oraclejdk8
+jdk: openjdk8
 before_install: gem install bundler -v '< 2'