Dropped oraclejdk8 from travis.yml.

Travis switched to Xenial which means that oraclejdk8 is no longer
available as an option in the jdk-list. So, to keep up with the latest
changes we need to drop Oracle JDK 8 from our builds.

Author: wcmatthysen

.travis.yml

@@ -10,7 +10,6 @@ jdk:
   - openjdk8
   - openjdk11
   - openjdk12
-  - oraclejdk8
   - oraclejdk11
   - openjdk-ea