Compatibility to Java 9

[jGleitz]

This library does not seem to be compatible to Java 9, (see https://travis-ci.org/robstoll/atrium/builds/575927779)

The error message is:

error: cannot access module-info

  bad class file: /home/travis/.gradle/caches/modules-2/files-2.1/ch.tutteli.niok/niok/0.3.0/eecfeb6c40253092a279e92c9618ddde052bb8b3/niok-0.3.0.jar(/module-info.class)

    class file has wrong version 55.0, should be 53.0

    Please remove or make sure it appears in the correct subdirectory of the classpath.

It seems like the target version should be set to at least Java 9 when compiling, but I am not sure about that.

[jGleitz]

I can only reproduce this with Java 9. With Java 10, this only becomes a warning:

warning: /home/josh/.gradle/caches/modules-2/files-2.1/ch.tutteli.niok/niok/0.3.0/eecfeb6c40253092a279e92c9618ddde052bb8b3/niok-0.3.0.jar(/module-info.class): major version 55 is newer than 54, the highest major version supported by this compiler.
  It is recommended that the compiler be upgraded.

So projects cannot be compiled with Java 9 if they have a dependency on this project.

However, since Java 9 is not supported anymore, it might not be worth “fixing” this bug and just advice clients (notably atrium) to use an up-to-date compiler.

[robstoll]

Oh well, seems like Java 9 (Version 53) supports only java 9 and 10 but not 11 (Version 55). I'll release a newer version compiled with 9 later this afternoon

[robstoll]

released 0.3.1 compiled with java 9