Test build against Java 9

[raquelalegre]

There's a new version of Java which was released this last summer. We found this out when trying to build with maven on Anastasis' laptop which has Java 9. We found the javadoc plugin was not compatible with Java 9, and the maven-compiler-plugin needs some extra configuration. @ageorgou knows more about this.

We'll need to:

• Set up tests in travis to run Java 9.
• Change pom.xml with Anastasis latest changes and make sure those work also in Java 8.
• Update README which has some references to Java 8 accordingly.

It doesn't have much priority for now since it only affected development work and not users.

[ageorgou]

A bit more detail: The change was specifying a newer version of maven-compiler-plugin, and explicitly giving source and target Java versions (Java 8 seems to be fine, so not sure why this wasn't needed before - it might have to do with a new Maven version?):

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <source>1.8</source>
        <target>1.8</target>
    </configuration>
    <version>3.7.0</version>
</plugin>

The same change is required in the pom.xml for the mavenjython plugin.

For full reference, even though these changes might not be needed in the final version: We also removed two things which were causing (more) errors (although the build is still not successful). In the nammu pom.xml, we removed jython-swingutils as a requirement. In the mavenjython pom.xml, we removed the maven-javadoc-plugin.

[ageorgou]

After looking into #302, it is still not possible to get it to work with Java 9. To clarify, this is only a problem for developers, not users - running Nammu with a JRE v9 is fine.

The issue might be with netty, a library for network connections used under the hood. Similar compatibility issues with Java 9 are reported by other users. This causes issues with installing pyoracc from GitHub, although other packages appear to be installed fine.