Maven report generation fails on Java 9

[cruftex]

Although with the latest plugin versions report generation fails (mvn site). It seams that findbugs is not updated to Java 9 (and maybe never will....). Upstream bug: gleclaire/findbugs-maven-plugin#60

     [java] Warning: Unknown version of Java
     [java] edu.umd.cs.findbugs.JavaVersionException: Could not parse Java version string: 9

I suggest we just document the problem in the readme and postpone the resolution to 1.2 or 2.0.

[eolivelli]

@cruftex consider moving to Spotbugs as Findbugs is dead

[gregrluck]

Looks like SpotBugs is still stabilising. This is only used by the mvn site goal which we don't use and only occurs on Java 9. Lets move this to 2.0 as it is not blocking anything.