Change travis build to use OpenJDK instead of Oracle

All builds with references to Oracle JDK11 breaks as Oracle appears
to be restricting headless downloads. Travis is also moving to remove
support for Oracle JDKs from install-jdk.sh

No work is required by students as their local builds on Oracle JDK
should behave similarly to OpenJDK.

.travis.yml

@@ -1,7 +1,7 @@
 language: java
 matrix:
   include:
-    - jdk: oraclejdk11
+    - jdk: openjdk11
 
 install:
   - ./config/travis/download_gradle_wrapper.sh