Using openjdk8 instead of oraclejdk8 in Travis

Oracle JDK 8 isn't supported by Travis anymore due to changes in JDK
licensing
diff --git a/.travis.yml b/.travis.yml
index aa9100b..cba068d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,7 @@
 
 jdk:
   - oraclejdk9
-  - oraclejdk8
+  - openjdk8
 
 before_install:
   - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1280x1024x16"