Update Travis JDK to JDK11.

This is an attempt to fix the error:

install-jdk.sh 2019-09-17
Expected feature release number in range of 9 to 14, but got: 8
The command "~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"" failed and exited with 3 during .

It is similar to CL 253584474

RELNOTES=n/a

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=272528889

.travis.yml

@@ -3,7 +3,7 @@ sudo: false
 language: java
 
 jdk:
-  - oraclejdk8
+  - oraclejdk11
 
 install: mvn install -U -DskipTests=true