Build using JDK9 on travis

[ktoso]

Just released gradle 4.1-milestone-1 will allow us to unblock progress on the JDK bridge.
Works locally, will proceed on making it work on travis.

Refs #294
Unblocks #296
Gradle release: https://github.com/gradle/gradle/releases/tag/v4.1.0-milestone-1

[ktoso]

FYI, the upgrade was done using the recommended method ./gradlew wrapper --gradle-version=4.1-milestone-1

[ktoso]

Stuck on attempting to get JDK9 on travis, gradle is ready ;<
travis-ci/travis-ci#5520

There's reports of people claiming to have gotten it working but I can't reproduce so far

[ktoso]

Thanks to some detective work by @TheSnoozer ( git-commit-id/git-commit-id-maven-plugin#292 (comment) ) I was able to get JDK9 running, now it's a question of making the build such that we have one separate project that builds only on 9, and we can prepare the bridge artifact then.

[ktoso]

See here https://travis-ci.org/reactive-streams/reactive-streams-jvm/builds/252653299?utm_source=github_status&utm_medium=notification

I'll continue working towards this in the coming days.

[akarnokd]

You may need a script entry export GRADLE_OPTS=-Xmx1024m to avoid the OOMKiller 137.

[ktoso]

Closing, will be done as part of the other PR introducing the adapters I think