Use a newer JDK 8 for travis build

The travis docker image uses JDK 8u31, which is ancient and has type
inference bugs that are fixed at head. This change causes it to use
8u111 instead:

Fixes google#2625

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138184449

.travis.yml

@@ -5,6 +5,12 @@ language: java
 jdk:
   - oraclejdk8
 
+# https://github.com/travis-ci/travis-ci/issues/3259#issuecomment-130860338
+addons:
+  apt:
+    packages:
+      - oracle-java8-installer
+
 install: mvn -B install -U -DskipTests=true
 
 # https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received