Travis: Add JDK 9 to matrix of CI builds

It seems we will have to remove the built-in support for Substance, which will be a separate issue.
As well as probably call the upcoming version 5.2 instead of 5.1.3.
So far my suggestion.

See also IZPACK-1185. This will also need changes in the Wiki - GUI Preferences.

Reasons:

• For each JDK we need a different version of Substance

• There are not all versions of Substance deployed to the Maven Central repositories, and those which there are are deployed by someone else than the developer as fork of the original sources.

I digged a bit more and found out that acctually it is the InstallationTest.testSubstanceLaf which errors for me. It seems that it errors due to Java 9, please see https://github.com/Insubstantial/insubstantial/issues/56.
While debugging I came to the same place as mochaman in his comment (3 Oct 2017).

So it seems, that Substance will not be available on Java 9 until we use a different version of the library or we patch it for ourselves.

@Adam Jurcik I would suggest to create a jdk9 branch on the main project and I will then create a PR containing my current changes....

The InstallationTest fails also for me localy on the current IzPack master.

You probably run on a different OS and /or other environment.
There is a discussion about this here.
Can you play a bit with the version of the Substance L&F library versions?
Original development site: https://github.com/kirill-grouchnikov/substance/releases
Unfortunately, in Maven Central there is only 7.3 as the latest version:

• https://mvnrepository.com/artifact/com.github.insubstantial/substance

• https://mvnrepository.com/artifact/com.github.insubstantial/substance-swingx

You might also recompile the latest one from the original Github repository.
Unfortunately the API changed.
I'm not sure Substance is the way for us in future or whether we will have to kick it out.

The InstallationTest fails also for me localy on the current IzPack master.