Add jdk9 build to travis

[kiview]

I've added a jdk9 build (basically the first compiling step) to Travis. I also wonder if we should add openjdk8 and openjdk9 to the compiling step, or maybe switch to openjdk from oraclejdk, but it might be not really important for us,

[bsideup]

this will double the required number of executors on Travis:(

I would rather add 1 stage like docker-in-alpine-docker but with openjdk:9-jdk-alpine

[kiview]

AFAIK this does not double the required number of executors, since it seems it's not applied as a matrix configuration in this case, but will only duplicate the

install:
  - echo "MAVEN_OPTS='-XX:+TieredCompilation -XX:TieredStopAtLevel=1'" > ~/.mavenrc
  - ./mvnw -T4 -DskipTests=true -Dmaven.javadoc.skip=true install

step and not the individual jobs. At least it looks like this in Travis.

[kiview]

I also wonder why this step runs tests on case of jdk9?

[kiview]

Okay, travis seems to have added a new default job for both JDKs instead of applying it to the build matrix, I'll try something else instead.

[kiview]

Btw. it seems like some Docker-Compose tests are failing with JDK9 🙀

[kiview]

Also ensured JDK9 compatibility by adding explicit dependency on javax.xml.bind.jaxb-api.

[rnorth]

LGTM