Add Java 9 to the Travis' build matrix

[plamentotev]

Java 9 is coming soon so it would be nice to have the project built and tested on Java 9 as well.

The change of the distro to trusty is required because it's the only one supporting Java 9 right now.

[plamentotev]

Hmm. Looks like it is not so easy 😄. JDK 7 build is failing. I'll take a look at it.

[plamentotev]

Looks like the Trusty image does not have JDK 7. Using it only for JDK 9 did the trick.

[rfscholte]

See https://docs.travis-ci.com/user/precise-to-trusty-migration-guide/
Trusty is the new default. You can still use Java 7, but you must refer to it is openjdk7

[plamentotev]

Trusty is the new default.

Actually they are rolling it out gradually. I was waiting the migration to be over before removing the explicit Trusty configuration. I've removed it and the build fails because Precise is still the default for this repository. Do you know if there is a way to request the repository to be migrated? For the time being I'll just set it explicitly.

You can still use Java 7, but you must refer to it is openjdk7

To be honest I'm not completely sure what is the exact difference between OpenJDK and the Oracle JDK. Some projects (like common-compress) actually test with both OpenJDK and Oracle JDK. Should we do the same here or they are close enough to test only on one of them.

[plamentotev]

Now the Enforcing plugin is failing. The parent pom should be updated to use the latest version, shouldn't it?