Build and test against multiple version of Java

TODO:
- Restrict impact on third-party/PR reporting tools, if applicable.

.travis.yml

@@ -1,11 +1,31 @@
 language: java
-jdk:
-  - oraclejdk8
+matrix:
+  include:
+    - env: JDK='Oracle JDK 8'
+      jdk: oraclejdk8
+    - env: JDK='Oracle JDK 9'
+      jdk: oraclejdk9
+    - env: JDK='Oracle JDK 10'
+      install: . ./install-jdk.sh -F 10 -L BCL
+    - env: JDK='OpenJDK 10'
+      install: . ./install-jdk.sh -F 10 -L GPL
+    - env: JDK='Oracle JDK 11'
+      install: . ./install-jdk.sh -F 11 -L BCL
+    - env: JDK='OpenJDK 11'
+      install: . ./install-jdk.sh -F 11 -L GPL
+  allow_failures:
+    # XXX: Error Prone is not yet compatible with JDK 11.
+    - env: JDK='Oracle JDK 11'
+    - env: JDK='OpenJDK 11'
 addons:
   sonarcloud:
     organization: picnic-technologies
     token: "${SONARCLOUD_TOKEN}"
 before_install:
+  # XXX: Direct usage of `install-jdk.sh` might be superseded by
+  # https://github.com/travis-ci/travis-build/pull/1347
+  - unset _JAVA_OPTIONS
+  - wget https://github.com/sormuras/bach/raw/1.0.1/install-jdk.sh
   - mvn io.takari:maven:wrapper
 install:
   - ./mvnw clean install -DskipTests