Upgrade Travis configuration to use a recent version of Oracle JDK 8

By default, Travis uses a rather old version of Oracle's JDK 8 in
its container-based infrastructure. It would appear that this is what
has been causing the build to fail with minimal diagnostics.

This commit implements a suggestion [1] to get a more recent version
of Java 8 by installing the oracle-java8-installer package via the APT
addon.

[1] travis-ci/travis-ci#3259 (comment)

Author: wilkinsona

Diff for: .travis.yml

@@ -1,6 +1,10 @@
 sudo: false
+addons:
+  apt:
+    packages:
+      - oracle-java8-installer
 language: java
 jdk:
   - oraclejdk8
 script:
-  - "./gradlew build buildSamples --stacktrace --info --no-daemon"
+  - "./gradlew build buildSamples --stacktrace --no-daemon"