CI: Use openjdk8 instead of oraclejdk8

Turbo87 · Turbo87 · commit 7bee2c79c132 · 2019-07-23T09:54:42.000+02:00
`oraclejdk8` won't install reliably anymore due to a change on the Oracle servers
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,5 @@
 language: java
-jdk:
-  - oraclejdk8
+jdk: openjdk8
 
 before_deploy:
   - ./gradlew buildPlugin
