Switch to OpenJDK 8 for Travis builds

This is the same issue as LiveRamp/pmd_extensions@997e753

> Travis no longer supports Oracle JDK 8, only 9 through 14. However, we
internally only use JDK 8 and so this code must work on feature set 8 so
we set this to the openjdk8 environment which should work.
>
> For examples from the community see:
https://travis-ci.community/t/error-installing-oraclejdk8-expected-feature-release-number-in-range-of-9-to-14-but-got-8/3766/3

Author: Roshan George

.travis.yml

@@ -1,6 +1,6 @@
 language: java
 jdk:
-- oraclejdk8
+- openjdk8
 notifications:
   email: false
 before_deploy: